Record why DISABLE_VERIFY_CERTIFICATE never worked

The daemon reads the key with seafile_session_config_get_bool, which is a
case sensitive g_strcmp0(value, "true"), so the capitalised True stored by
released images up to 0.0.13 was read as false and verification stayed on for
anyone who asked to disable it. The default was never affected, since any
value other than "true" is false.

Rendering booleans in lower case fixed this; this only writes down what the
change actually repaired, for anyone who reported the flag as broken.

IMPROVEMENTS.md

@@ -95,8 +95,12 @@ Acceptance criteria:
   they were compared against, so `usermod` ran on every start.
 - [x] Reject negative and malformed numeric values with useful messages.
 - [x] Do not pass `None` as a Seafile configuration value. Booleans are also
-  rendered as `true`/`false` rather than Python's `True`/`False`, since the
-  daemon reads that setting with its boolean accessor.
+  rendered as `true`/`false` rather than Python's `True`/`False`. This fixed
+  `DISABLE_VERIFY_CERTIFICATE`, which never worked: the daemon reads the key
+  with `seafile_session_config_get_bool`, which is
+  `g_strcmp0(value, "true") == 0`, so the stored `True` was false and
+  verification stayed on for anyone who asked to disable it. The default was
+  never affected, because anything other than `true` is false.
 
 Acceptance criteria: