docs/library/ure: Describe (lack of) escaping in character sets.
docs/library/ure: Describe (lack of) escaping in character sets.

Only `\]` escape is supported.

Change-Id: I3e340fc0d42f395fb6ea4e2ae08e72af74103fbb
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython