Update docs for Zim spell checker performance hints

[vossenjp]

Opened per @introt's request in #2167 (comment).
Details in in #1497 (comment) or #2167 (comment) (same details in both).

Duplicated here for quick reference.

tl;dr: If Zim is very slow or if the spell checker is misbehaving, try removing gtkspellcheck and using gtkspell instead. For Ubuntu, Mint, etc.:

• sudo apt purge python3-enchant python3-gtkspellcheck
• sudo apt install gir1.2-gtkspell3-3.0 libgtkspell3-3-0
• sudo add-apt-repository ppa:jaap.karssenberg/zim && sudo apt update && sudo apt install zim

Details:

I've had various problems with the spell checker (which I require :), e.g. #2167, #1497, and maybe #1596, which I believe all started with the move to Python3. I've the same problems on multiple machines, running Mint-19.x, 20.x and 21.0, with various resources. In particular, they persisted when I built a brand new 16 core Ryzen 7 which should not present any performance problems. Related to performance, note I do keep Zim wiki files on a remote SSH FileSystem mount, but it's a local 1G LAN.

I had the "ah ha" moment when I build a brand new Mint-21.0 VM for work, installed Zim from PPA, and it Just Worked Great, with none of the problems. When I looked into it, I saw "Edit > Preferences > Plugins > Spell Checker: gtkspell - OK (gtkspellcheck - Failed)". Meanwhile, all the bad ones had "gtkspellcheck - OK" I assume for some historical reason or other. When I switched a bad one from gtkspellcheck to gtkspell all the problems went away!

I did not specifically install either gtkspellcheck or gtkspell on this new VM, so I assume that gtkspell is (now?) the default somewhere. So I am thinking that the move to Python3 somehow broke gtkspellcheck in a Zim context? And maybe since gtkspell seems to be the default now, that's why most folks don't have this problem?

This also explains why turning spell checking off (#1497) "fixes" the problem.

[introt]

Thanks!

This may require some further investigation, as gtkspellcheck aka koehlma/pygtkspellcheck (packaged as python3-gtkspellcheck) should be the newer one - the latest release of libgtkspell3-3-0 (version 3.0.10-1) is from 2018 according to the project's homepage, https://gtkspell.sourceforge.net/

Edit: Well, newer and newer, gtkspellcheck 3.0 does date back to 2012. Some digging is indeed needed, though an API change/regression is the most likely cause.