cryptsetup-generator: allow overriding /run/systemd/cryptsetup with $RUNTIME_DIRECTORY

I added a fairly vague entry to docs/ENVIRONMENT because I think it is worth
mentioning there (in case someone is looking for any environment variable that
might be relevant).

docs/ENVIRONMENT.md

@@ -64,6 +64,10 @@ All tools:
   this only controls use of Unicode emoji glyphs, and has no effect on other
   Unicode glyphs.
 
+* `$RUNTIME_DIRECTORY` — various tools use this variable to locate the
+  appropriate path under /run. This variable is also set by the manager when
+  RuntimeDirectory= is used, see systemd.exec(5).
+
 systemctl:
 
 * `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus