Update gmt.conf.rst

[gd-a]

Description of proposed changes

Updates the gmt.conf documentation so that the defaults values are all included in brackets and the allowed (non-numeric) values for each parameter are bolded.

Fixes #4820
Fixes #4822

updated up to IO_SEGMENT_MARKER

[maxrjones]

Thanks for starting on this, @gd-a! For future PRs, please include a full description of the proposed changes (even if they are already detailed in an issue). I've edited the description of this PR as an example. Full descriptions in the PRs are helpful because they simplify tracking of changes and make it easier for people who get email notifications about new pull requests. The fixes #____ is particularly helpful because it enables GitHub to automatically close issues when PRs get merged into the code base.

[gd-a]

I think I submitted the full version

[maxrjones]

L86: change from
are: **none** (default: use whatever the **COLOR_MODEL** setting in the
to
to are **none** (use whatever the **COLOR_MODEL** setting in the

[joa-quim]

[default is black])

Yes, agree, 'default is' should be normal font. No point in italicize it.

[maxrjones]

[default is black])

Yes, agree, 'default is' should be normal font. No point in italicize it.

@gd-a, please replace the instances of *default is* with default is.

[gd-a]

@meghanrjones : all your suggestions have been committed.

[maxrjones]

This is great, thanks @gd-a! The only parameter that remains different from the others is COLOR_MODEL. It should be good to go after that is updated. If there's a reason why you think it should stay as is, please feel free to post.

[gd-a]

The only parameter that remains different from the others is COLOR_MODEL.

How do you want it ?

[maxrjones]

The only parameter that remains different from the others is COLOR_MODEL.

How do you want it ?

See this comment: https://github.com/GenericMappingTools/gmt/pull/4834/files#r579679911 and this suggestion: https://github.com/GenericMappingTools/gmt/pull/4834/files#r579680017

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