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.

gdb/findcmd.c

@@ -293,7 +293,9 @@ and if not specified the size is taken from the type of the expression\n\
 in the current language.\n\
 Note that this means for example that in the case of C-like languages\n\
 a search for an untyped 0x42 will search for \"(int) 0x42\"\n\
-which is typically four bytes.\n\
+which is typically four bytes, and a search for a string \"hello\" will\n\
+include the trailing '\\0'.  The null terminator can be removed from\n\
+searching by using casts, e.g.: {char[5]}\"hello\".\n\
 \n\
 The address of the last match is stored as the value of \"$_\".\n\
 Convenience variable \"$numfound\" is set to the number of matches."),