[doc] improve MI varobj introduction

[Vladimir Prus <vladimir at codesourcery dot com>]

Hello,
I think that at the moment, the introduction for varobj has a number of problems:

	- It does not mention update operation, which is 50% of varobjs relevance
	- It does not adequately describe hierarchy of varobjs, in particular that
	only leaf varobjs hold real data and can be manipulated.
	- There's a paragraph about hex/decimal format, which is minor
	detail and need not be mentioned.
	- The grammar of the text is overly complex sometimes.

The attached patch basically rewrites the introduction. The semantic of -var-update
is also clarified. The new introduction is basically one blob of text. There are places
where docbook <firstterm> would be appropriate, but it looks to be not supported
in info.

Comments?

- Volodya

	* gdb.texinfo (GDB/MI Variable Objects): Improve the 
	introduction.  Specify -var-update more exactly.

--- gdb/doc/gdb.texinfo	(/mirrors/gdb_mainline)	(revision 2727)
+++ gdb/doc/gdb.texinfo	(/patches/gdb/varobj_doc/gdb_mainline)	(revision 2727)
@@ -19570,26 +19570,36 @@ least, the following operations:
 
 @end ignore
 
-@subheading Introduction to Variable Objects in @sc{gdb/mi}
+@subheading Introduction to Variable Objects
 
 @cindex variable objects in @sc{gdb/mi}
-The basic idea behind variable objects is the creation of a named object
-to represent a variable, an expression, a memory location or even a CPU
-register.  For each object created, a set of operations is available for
-examining or changing its properties.
-
-Furthermore, complex data types, such as C structures, are represented
-in a tree format.  For instance, the @code{struct} type variable is the
-root and the children will represent the struct members.  If a child
-is itself of a complex type, it will also have children of its own.
-Appropriate language differences are handled for C, C@t{++} and Java.
-
-When returning the actual values of the objects, this facility allows
-for the individual selection of the display format used in the result
-creation.  It can be chosen among: binary, decimal, hexadecimal, octal
-and natural.  Natural refers to a default format automatically
-chosen based on the variable type (like decimal for an @code{int}, hex
-for pointers, etc.).
+
+Variable object is MI interface to work with expressions.  When
+creating a variable object, a frontend specifies the name of
+the variable object and an expression in the debugged program.
+Such expression can be a simple variable, or it can be an arbitrary
+complex expression, and can even involve CPU registers.  After
+creating a variable object, a frontend can invoke other variable
+object operations -- for example to obtain or change the value of
+a variable object, or to change format.
+
+Variable objects have hierarchical structure.  Any variable object
+that corresponds to a composite type, such as structure in C, has
+a number of child variable objects, for example corresponding to each
+element of a structure.  Child variable objects can children themself,
+util we reach leaf variable objects of built-in types.
+
+For a leaf variable object it is possible to obtain its value as a
+string, or set the value from a string.  String value can be also
+obtained for a non-leaf variable object, but its generally a string
+that only indicates the type of the object, and does not list its
+content.  Assignment to a non-leaf variable object is not allowed.
+ 
+A frontend need not read the values of all variable objects each time
+the program stops.  Instead, MI provides update command that lists all
+variable objects which values has changed since the last update
+operation.  This considerably reduces the amount of data that must
+be transferred to the frontend.
 
 The following is the complete set of @sc{gdb/mi} operations defined to
 access this functionality:
@@ -19888,12 +19898,19 @@ subsequent @code{-var-update} list.
  -var-update [@var{print-values}] @{@var{name} | "*"@}
 @end smallexample
 
-Update the value of the variable object @var{name} by evaluating its
-expression after fetching all the new values from memory or registers.
-A @samp{*} causes all existing variable objects to be updated.  The
-option @var{print-values} determines whether names both and values, or
-just names are printed in the manner described for
-@code{-var-list-children} (@pxref{-var-list-children}).
+Reevaluate the expressions corresponding to the variable object
+@var{name} and all its direct and indirect children, and return the
+list of variable object which values has changed as the result. Here,
+``changed'' means that the result of @code{-var-evaluate-expression} before
+and after the @code{-var-update} is different.  If @samp{*} is used
+as the variable object names, all existing variable objects are
+updated.  The option @var{print-values} determines whether both names 
+and values, or just names are printed.  The possible values of
+this options are the same as for @code{-var-list-children}
+(@pxref{-var-list-children}).  It is recommended to use the
+@samp{--all-values} option, to reduce the number of MI commands needed
+on each program stop.
+
 
 @subsubheading Example