doc: add configuration section to notmuch-insert(1)

This is partially redudant given some existing cross references, but
it is useful to have all of the config keys listed in one place, to
help keep track of them if nothing else.

doc/man1/notmuch-insert.rst

@@ -88,6 +88,16 @@ Supported options for **insert** include
 
    See also ``index.decrypt`` in :any:`notmuch-config(1)`.
 
+CONFIGURATION
+=============
+
+Indexing is influenced by the configuration options
+:ref:`index.decrypt <index.decrypt>` and :ref:`index.header
+<index.header>`.  Tagging
+is controlled by :ref:`new.tags <new.tags>` and
+:ref:`maildir.synchronize_flags <maildir.synchronize_flags>`.  See
+:any:`notmuch-config(1)` for details.
+
 EXIT STATUS
 ===========