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 firstname.lastname@example.org