Update CAPTCHA_SETUP.rst X-Forwarded-For docs
Update CAPTCHA_SETUP.rst X-Forwarded-For docs

It looks like CAPTCHA_SETUP.rst contains information relevant to an old version of Synapse, but Synapse now has a different approach to configuring use of the X-Forwarded-For header.
matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse