[RFC] Docs: Remove 'shortname' references

#635 Didn't touch the docs, which this PR does. I didn't attempt to remove the remaining `shortname` references in `{option,path,fileio,eval}.c` as I'm not competent enough in C.

927c623 is separate as it also includes the removal of `key` and `cm` (cryptmethod), which I missed in #1563. Due to the incredibly long lines in 927c623, it might be easier to parse if locally using `git show --word-diff`.