prefix_filename: move docstring to header file

This is a public function, so we should make its
documentation available near the declaration.

While we're at it, we can give a few details about how it
works.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

abspath.c

@@ -246,11 +246,6 @@ char *absolute_pathdup(const char *path)
 	return strbuf_detach(&sb, NULL);
 }
 
-/*
- * Unlike prefix_path, this should be used if the named file does
- * not have to interact with index entry; i.e. name of a random file
- * on the filesystem.
- */
 const char *prefix_filename(const char *pfx, int pfx_len, const char *arg)
 {
 	static struct strbuf path = STRBUF_INIT;