Radec warnings

[araichoor]

This PR:

• addresses Add warnings to files that contain (TARGET|PLATE|FIBER)_(RA|DEC) or (PLATE_|)REF_EPOCH.  #168; the list of modifications is summarized here: Add warnings to files that contain (TARGET|PLATE|FIBER)_(RA|DEC) or (PLATE_|)REF_EPOCH.  #168 (comment);
• sneaks in a small addition of "optional columns" for doc/DESI_ROOT/vac/RELEASE/lss/VERSION/inputs_wspec/data/datcomb_tarspecwdup.rst (as those columns do not exist in the dark version of the files).

Documentation with the current version of this PR is available here: https://data.desi.lbl.gov/desi/users/raichoor/tmpdir/desidatamodel_radec_warnings/doc/v0/.

[coveralls]

coverage: 100.0%. remained the same when pulling 4c3c7c9 on radec_warnings into ec332b6 on main.

[weaverba137]

I don't see any problems with this. Free to merge when ready.

[araichoor]

Thanks; I m making a final check with looking at the modified html files one by one.

[araichoor]

I just pushed a commit removing the "fiber_radec pmcorr warning" for one file, as it actually doesn t contain the FIBER_{RA,DEC} columns.

I ll merge when the checks have passed.