setup.c: document get_pathspec()

Since we have just created a new pathspec-handling library, now is a
good time to add some comments explaining get_pathspec().

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -249,6 +249,25 @@ static const char *prefix_pathspec(const char *prefix, int prefixlen, const char
 		return prefix_path(prefix, prefixlen, copyfrom);
 }
 
+/*
+ * N.B. get_pathspec() is deprecated in favor of the "struct pathspec"
+ * based interface - see pathspec_magic above.
+ *
+ * Arguments:
+ *  - prefix - a path relative to the root of the working tree
+ *  - pathspec - a list of paths underneath the prefix path
+ *
+ * Iterates over pathspec, prepending each path with prefix,
+ * and return the resulting list.
+ *
+ * If pathspec is empty, return a singleton list containing prefix.
+ *
+ * If pathspec and prefix are both empty, return an empty list.
+ *
+ * This is typically used by built-in commands such as add.c, in order
+ * to normalize argv arguments provided to the built-in into a list of
+ * paths to process, all relative to the root of the working tree.
+ */
 const char **get_pathspec(const char *prefix, const char **pathspec)
 {
 	const char *entry = *pathspec;