Correct out-of-date passages in the man pages

[brzzdev]

Several passages in task(1) and taskrc(5) no longer match what Taskwarrior 3.5.0 does.

• _get exits with 2 for an invalid DOM reference. A missing value prints nothing and exits with 0; the page said both exit with 1.
• sow, som, soq and soy are the start of the current period, not the next one.
• The or example said the command line filter needs parentheses to combine correctly with the report filter. Taskwarrior now adds them itself. The section now explains that, and gives an example where parentheses are still needed, when mixing and and or.
• socw and eocw were never recognised in 3.x, but they were still in the man page and the zsh and fish completions. I removed them, as discussed in 2.6.X: 'eocw' and 'socw' are invalid #3628, and the week headings now say which day each date falls on. soww/eoww refer to the next work week when used on a Sunday.
• weekstart only changes the calendar layout, week numbers and week dates such as 2026-W02. The taskrc(5) entry now says that it does not affect sow, eow or the WEEK virtual tag.

I checked each change against the source and the 3.5.0 binary with an empty database.

The _get change documents current behaviour. #4196 left open whether a missing value exiting with 0 is intended. If it's a bug, this paragraph should change with the fix.

The website's DOM page had the same tag.<name> problem reported in #4196, and it's fixed separately in GothenburgBitFactory/tw.org#1465.

Closes #4196
Closes #3628

[djmitche]

Thanks for these fixes!

I want to clarify for those following along that these represent changes that have occurred sometime in the last ~5 years, and for which documentation wasn't properly updated. 3.0.0 was released during that 5-year period, but these changes weren't specifically associated with that release.