Update Help/Menu/Imagery#Caching offline strategy discussion

[jidanni]

https://josm.openstreetmap.de/wiki/Help/Menu/Imagery#Caching says

To reuse the tiles over JOSM sessions without having the reload them
from the server, right click on the WMS layer and set a bookmark....
... you can work offline using this technique.

OK but that paragraph might need to be updated as it seems these days
one does not need to go through such trouble, as one can still see the
cached imagery offline anyway.

(By the way, the goal of a real offline mode would be:

No more hopeless hammering of the "network" (that isn't there. We are in
a cabin in the woods) trying to get new / fresher imagery tiles.

Have the program face the facts and just be content to use the tiles
already in the cache.

(But do also document the special case of though being online, wanting
one layer of tiles only come from a disk cache and not refresh from the
network.)

Anyway when offline our concern is using what we already have, without
any danger of good existing tiles expiring or being replaced with blanks
due to no internet connection.

Anyway, this is just one of the areas that needs to be tested to make
JOSM fully "offline ready".

And yes, it seems wise to have an explicit "Offline mode" available in
the main menus, like Firefox, instead of current JOSM and chromium
browser behavior.)

[stoecker]

Feel free to improve documentation. It's a wiki.