Add most of AnnData merged docs

[sjspielman]

Towards #241 This is the first of two PRs (second coming after https://github.com/AlexsLemonade/scpca-nf/issues/681 is wrapped up) to fill in the rest of the AnnData docs for merged objects.

Note that I have one !!TODO! little bit in there since whether we end up with suspension_type (vs just keeping it as seq_unit) depends on that https://github.com/AlexsLemonade/scpca-nf/issues/681 as well. I imagine we'll be keeping it as seq_unit since we don't need CZI compliance for merged objects?

There are also a few corrections I made along the way to SCE merged objects that I noticed:

• We were missing the additional_modalities column in colData
• The metadata(merged_sce)$library_metadata$SCPCL000000 list was missing a few fields, and it also incorrectly listed sample_metadata which was removed.
• Fixed a couple typos, also in sce_object_contents.md where the same typo was in both files.
• Also, in sce_object_contents.md, suspension_type was missing from the AnnData obs slot, and it indeed appears to be in those objects (checked on in scpca-prod).

[sjspielman]

https://github.com/AlexsLemonade/scpca-nf/issues/681 is closed, so re-requesting review now.