docs: Drop use of `<!-- -->` to delimit plurals

gtk-doc gained the ability to magically strip off the ‘s’ in a plural many years ago, so the workaround of splitting the ‘s’ from a class or function name is no longer needed.

This clarifies the docs in the code a bit.

This commit was made mechanically using

git search-replace -f '<!-- -->s///s'

Signed-off-by: Philip Withnall withnall@endlessm.com