crypt: warn about directories with legacy version-like encrypted names

Directory names which look like they have a --b2-versions version
string are now encrypted in full, so directories created by older
rclone (which left the version string in plain text) no longer
decrypt and vanished silently from listings.

DecryptDirName now falls back to the old form for such names so the
directory is listed, and logs the name it needs to be renamed to on
the underlying remote to make it accessible again. Document this in
the crypt docs.

docs/content/crypt.md

@@ -813,6 +813,15 @@ This means that
 - filenames with the same name will encrypt the same
 - filenames which start the same won't have a common prefix
 
+A version string of the form `-vYYYY-MM-DD-HHMMSS-NNN` on the end of a
+file name (as added by `--b2-versions` / `--s3-versions`) is left in
+plain text so that versioned files can be found. Directory names are
+encrypted in full. Rclone before v1.76 left such a suffix in plain
+text on directory names too, so a directory named like this created by
+an older rclone will appear in listings with a warning but can't be
+opened or removed until renamed on the underlying remote to the name
+given in the warning.
+
 This uses a 32 byte key (256 bits) and a 16 byte (128 bits) IV both of
 which are derived from the user password.