(Starting GUD): Add an entry for gud-gdb.

(GDB Graphical Interface): Explain that gud-gdb is now needed for text
command mode.

doc/emacs/building.texi

@@ -477,9 +477,15 @@ to a particular debugger program.
 @table @kbd
 @item M-x gdb @key{RET} @var{file} @key{RET}
 @findex gdb
-Run GDB as a subprocess of Emacs.  By default, this uses an IDE-like
-graphical interface; see @ref{GDB Graphical Interface}.  Only GDB
-works with the graphical interface.
+Run GDB as a subprocess of Emacs.  This uses an IDE-like graphical
+interface; see @ref{GDB Graphical Interface}.  Only GDB works with the
+graphical interface.
+
+@item M-x gud-gdb @key{RET} @var{file} @key{RET}
+@findex gud-gdb
+Run GDB as a subprocess of Emacs.  This command creates a buffer for
+input and output to GDB, and switches to it.  If a GDB buffer already
+exists, it just switches to that buffer.
 
 @item M-x dbx @key{RET} @var{file} @key{RET}
 @findex dbx
@@ -829,14 +835,9 @@ unlimited, so it sets the height and width accordingly.  For correct
 operation you must not change these values during the GDB session.
 
 @vindex gud-gdb-command-name
-  You can also run GDB in text command mode, like the other debuggers
-in Emacs.  To do this, replace the GDB @code{"--annotate=3"} option
-with @code{"--fullname"} either in the minibuffer for the current
-Emacs session, or the custom variable @code{gud-gdb-command-name} for
-all future sessions.  You need to use text command mode to debug
-multiple programs within one Emacs session.  You can also use
-@kbd{M-x gud-gdb} to invoke GDB in text command mode if you have
-problems before execution has started.
+  To run GDB in text command mode, like the other debuggers in Emacs,
+use @kbd{M-x gud-gdb}.  You need to use text command mode to debug
+multiple programs within one Emacs session.
 
 @menu
 * GDB-UI Layout::               Control the number of displayed buffers.