doc: add note about process-unmapped-keys to unmod (jtroo#1228)

It has taken me a while to figure out why `unmod` had no effect in my
mappings. After running in debug mode I realized that the events sent by
the modifiers are not tracked at all, since they are not part of my
source mapping. When seeing this I remembered seeing
`process-unmapped-keys` enabled in [this comment](jtroo#615 (comment))
while digging through some issues. Doing so resulted in the expected
behavior.

docs/config.adoc

@@ -1566,6 +1566,10 @@ A variant of `unmod` is `unshift` or `un⇧`.
 This action only releases the `lsft,rsft` keys.
 This can be useful for forcing unshifted keys while AltGr is still held.
 
+NOTE: In case the modifiers to be undone are not part of `defsrc`,
+<<process-unmapped-keys>> needs to be enabled in `defcfg` in order for their
+states to be tracked correctly.
+
 .Example:
 [source]
 ----