Update lyx-2.4 docs

[Jean-Pierre Chrétien]

Dear Developers,

I performed a first update of the French 2.4 documentation around 20 months ago.

I would like to search easily what has been updated since (but for the new index 
features that I have  already updated in the UserGuide).
Is it possible to search for modifications after a given date in the documents ?
I could search for changes in Trac, but is is quite lengthy.

-- 
Regards
Jean-Pierre