Problem:
Descriptions are somewhat vague. nvim_set_current_line modifies contents
but nvim_set_current_buf does not, etc.
Solution:
- Make it clear that these functions accept or return a winid/tabid by
linking to that concept in help.
- Only these few files use the term "handles", so replace them with the
more conventional terminology.
- Add a new help section for tab-ID. This concept is unique to neovim
because vim exposes tabnr, but not tab handles. This section is
modelled after `:h winid`.
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