Add documentation for PIKA_THREADS, PIKA_IGNORE_PROCESS_MASK, and PIKA_BIND

[msimberg]

Based on #1380, merge that before this. Only the last commit is relevant for this PR.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.04% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (ac866b1)	18223	13735	75.37%
Head commit (4cc5062)	18223 (+0)	13727 (-8)	75.33% (-0.04%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1387)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[RMeli]

Thanks @msimberg, LGTM!