buffer: clarify how git_buf_sanitize handles non-NULL input

When `git_buf_sanitize` gets called, it converts a buffer with NULL
content to be correctly initialized. This is done by pointing it to
`git_buf__initbuf`. While the method's documentation states this
clearly, it may also lead to the conclusion that it will do the same to
buffers which do _not_ have NULL contents.

Clarify behavior when passing a buffer with non-NULL contents, where
`git_buf_sanitize` will ensure that the contents are `\0`-terminated.

src/buffer.h

@@ -66,7 +66,8 @@ extern int git_buf_try_grow(
  * library, when providing git_buf's, may wish to provide a NULL ptr for
  * ease of handling.  The buffer routines, however, expect a non-NULL ptr
  * always.  This helper method simply handles NULL input, converting to a
- * git_buf__initbuf.
+ * git_buf__initbuf. If a buffer with a non-NULL ptr is passed in, this method
+ * assures that the buffer is '\0'-terminated.
  */
 extern void git_buf_sanitize(git_buf *buf);