Document inclusive range in help for "find"

PR gdb/16930 points out that the "find" command uses an inclusive
range; but this is not documented in the "help".  This patch updates
the help text.

gdb/ChangeLog
2020-10-07  Tom Tromey  <tromey@adacore.com>

	PR gdb/16930:
	* findcmd.c (_initialize_mem_search): Mention that the range is
	inclusive.