Update find command help and search memory docs

Update find command help and search memory docs

This patch updates the `find` command help and docs description to show
how to search for not null terminated strings when current language's
strings includes it.

gdb/ChangeLog:

	PR gdb/21945
	* findcmd.c (_initialize_mem_search): Update find command help
	text.

gdb/doc/ChangeLog:

	PR gdb/21945
	* gdb.texinfo (Search Memory): Update description and example
	about how to search a string without NULL terminator.