reverse execution part of the manual is written backwards

[Paul Eggert <eggert@cs.ucla.edu>]

[resent from https://sourceware.org/bugzilla/show_bug.cgi?id=24417]

At least two Emacs developers were confused by GDB's documentation for reverse 
execution. One said he could never get it to work and recommended another 
debugger instead since it always worked for him. We tracked down the issue to a 
problem in GDB's documentation: it documents how to do reverse execution, but 
never mentions until a later section that you can't use reverse execution unless 
you first turn on process recording.

Surely it's not intended that one must read the GDB documentation backwards in 
order to know how to do reverse execution....

Proposed patch follows. This is just a minor doc patch so I assume no ChangeLog 
entry is needed.

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index f410d026b8..147c7c0f37 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -6697,8 +6697,11 @@ assumes that the memory and registers that the target 
reports are in a
  consistant state, but @value{GDBN} accepts whatever it is given.
  }.

-If you are debugging in a target environment that supports
-reverse execution, @value{GDBN} provides the following commands.
+Before using reverse execution, you should first use the @code{record}
+command, so that instructions executed by the program are saved for
+reverse execution later.  @xref{Process Record and Replay}.
+@value{GDBN} provides the following commands to examine the process
+record and execute the program in reverse.

  @table @code
  @kindex reverse-continue