oslo config documentation should mention the delimiters needed when using a '$' within a string value

[Keith Berger]

The oslo config code treat treats the '$' as a special character and it needs to be delimited when using within a string. This is mentioned in the code

https://github.com/openstack/oslo.config/blob/master/oslo_config/cfg.py#L2752-L2758

but not in any of the documentation

https://docs.openstack.org/oslo.config/latest/index.html

A note should be added to the documentation explain what is needed when using a $ within a string.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/724867
Committed: https://git.openstack.org/cgit/openstack/oslo.config/commit/?id=df5e5db191c13198a12b3eb24f0eda603e3324d8
Submitter: Zuul
Branch: master

commit df5e5db191c13198a12b3eb24f0eda603e3324d8
Author: Stephen Finucane <email address hidden>
Date: Fri May 1 16:10:30 2020 +0100

    docs: Mention other ways of escaping '$'

    oslo config code treat treats the '$' as a special character and it
    needs to be delimited when using within a string. There are two ways of
    doing this, '$$' or '\$'. We document the former but not the latter. Fix
    this.

    Change-Id: I37cdf31ba1c2924779a46b1a2eeabb66c46a2212
    Signed-off-by: Stephen Finucane <email address hidden>
    Closes-Bug: #1872839