api-strbuf.txt: add docs for _trim and _ltrim

API documentation for strbuf does not document strbuf_trim() or
strbuf_ltrim(). Add documentation for these two functions.

Signed-off-by: Brian Gesiak <modocache@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-strbuf.txt

@@ -121,10 +121,19 @@ Functions
 
 * Related to the contents of the buffer
 
+`strbuf_trim`::
+
+	Strip whitespace from the beginning and end of a string.
+	Equivalent to performing `strbuf_rtrim()` followed by `strbuf_ltrim()`.
+
 `strbuf_rtrim`::
 
 	Strip whitespace from the end of a string.
 
+`strbuf_ltrim`::
+
+	Strip whitespace from the beginning of a string.
+
 `strbuf_cmp`::
 
 	Compare two buffers. Returns an integer less than, equal to, or greater