strbuf: clarify API boundary

strbuf, as a generic and widely used structure across the codebase,
should be limited as a library to only interact with primitives. Add
documentation so future functions can appropriately be placed. Older
functions that do not follow this boundary should eventually be moved or
refactored.

Signed-off-by: Calvin Wan <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

strbuf.h

@@ -1,6 +1,14 @@
 #ifndef STRBUF_H
 #define STRBUF_H
 
+/*
+ * NOTE FOR STRBUF DEVELOPERS
+ *
+ * strbuf is a low-level primitive; as such it should interact only
+ * with other low-level primitives. Do not introduce new functions
+ * which interact with higher-level APIs.
+ */
+
 struct string_list;
 
 /**