improve doc of IndexSeries

[h-mayorquin]

Should fix #605

Checklist

For all schema changes:

• Add release notes for the PR to docs/format/source/format_release_notes.rst.
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.
• Make sure that hdmf-common-schema points to the latest release and not the latest commit on the main branch.

[h-mayorquin]

Here is my attempt at improving the docs, @rly .

[rly]

This is great. Thank you @h-mayorquin