mediawiki.loader: Better document IE stylesheet limit bug

Also leave a comment on the line where the relevant exception
will be thrown from if it does end up hitting the limit still
(which wouldn't be due to mw.loader appending more than 31
stylesheets but due to there being more than 30 on the page
already).

Change-Id: I3a46affc5498fbaa1d442ca65cb4844dd8f8169c
Merge "mediawiki.loader: Better document IE stylesheet limit bug"