Enhanced PITR doc patch

A range of minor enhancements, plus some stronger doc around:

- warnings about overwrites
- additional documentation of recovery.conf parameters

To be frank, its still about 50 pages short of a full discussion of how
it works, when to use it, various scenarios, best practice, integration
with XA (or current lack of it), tuning etc... all of which we can
expect to improve over time with help from the community.

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Best Regards, Simon Riggs