users/ignoring: #include uses relative paths (#807)

Specified/Clarified what happens (from my observation of behaviour) when
recursive `#include` directives are used.

Author: sapient-cogbag

users/ignoring.rst

@@ -80,6 +80,9 @@ The ``.stignore`` file contains a list of file or path patterns. The
    still relative to the synced folder *root*. Example:
    ``#include more-patterns.txt``.
 
+   Any ``#include`` directives inside a file loaded by ``#include`` require paths specified relative 
+   to the directory containing the loaded file, rather than the synchronised root directory.
+
 -  A pattern beginning with a ``!`` prefix negates the pattern: matching files
    are *included* (that is, *not* ignored). This can be used to override
    more general patterns that follow.