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;

cache.h

@@ -529,7 +529,19 @@ extern const char *setup_git_directory_gently(int *);
 extern const char *setup_git_directory(void);
 extern char *prefix_path(const char *prefix, int len, const char *path);
 extern char *prefix_path_gently(const char *prefix, int len, int *remaining, const char *path);
+
+/*
+ * Concatenate "prefix" (if len is non-zero) and "path", with no
+ * connecting characters (so "prefix" should end with a "/").
+ * 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.
+ *
+ * The return value may point to static storage which will be overwritten by
+ * further calls.
+ */
 extern const char *prefix_filename(const char *prefix, int len, const char *path);
+
 extern int check_filename(const char *prefix, const char *name);
 extern void verify_filename(const char *prefix,
 			    const char *name,