Add a description of a known Orca issue with Firefox, small fixes

[yhontyk]

• added a solution to a common issue of Orca not reading the content of webpages -- I am not sure if this is ideal placement as of now but it made sense grouping it with other improvements; it is not necessarily and improvement of the experience as much as it just solves a particular known issue
• some spelling fixes, added exceptions to the wordlist
• replaced a vague OrcaModifier with CapsLock and Insert as the two default options for Desktop

[yhontyk]

@msuchane the linkchecker seems to fail specifically for Matrix links. Do you want me to add these as exceptions?

[msuchane]

Reviewed, looks great. Thanks a lot @yhontyk!

If you feel like adding an exception for the Matrix link, I'd appreciate it, but no worries if not.

I'll merge the branch right after.