update conversion documentation

This adds some documentation, including a simple example to the prior constraints and slightly updates the documentation of the parameter conversions, including listing the places in the code where these are used.

Is the prior-constrains docs linked somewhere from the other docs? E.g from the index (https://git.ligo.org/lscsoft/bilby/blob/master/docs/index.txt) or perhaps the prior page?

added 10 commits

• 8764af78...6b49b514 - 8 commits from branch master
• 804cc528 - Merge branch 'master' into constraint-documentation
• 21c7a6fc - move prior constraint documentation

Compare with previous version

I moved the new documentation into the prior documentation.

approved this merge request

changed milestone to %0.4.5

approved this merge request

merged

mentioned in commit 971deb31