improve std.string.strip documentation

From the thread:
Java moves to copying for substrings
over at:
http://forum.dlang.org/thread/rnogwmgunahampftnzvs@forum.dlang.org
Specifically the comment:
"avoid memory leakage caused by retained substrings holding the entire character array."

Technically, most string/array/range based functions are subject to this "issue", but I figure strip is more vulnerable to it, and is the ideal place to document  and raise awareness of the the "a slice holds on to the entire array" issue.