gdb: update comment for execute_command_to_string

The function execute_command_to_string had two header comments, one in
gdbcmd.h and one in top.c.

This commit merges the two comments into one and places this comment
in gdbcmd.h.  The comment in top.c is updated to just reference
gdbcmd.h.

gdb/ChangeLog:

	* gdbcmd.h (execute_command_to_string): Update comment.
	* top.c (execute_command_to_string): Update header comment.