docs(all): clarifies what is overridden on a per-api basis

[HCloward]

What/Why/How?

There is some confusion with users about how per-API rules, decorators, and preprocessors merge with root settings. When there are per-API rules, decorators, and preprocessors, we merge with the top-level root properties. This merging does not include parameters specific to the rules, decorators, or preprocessors.

Reference

Relates to #1871

Security

• Security impact of change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

⚠️ No Changeset found

Latest commit: 0e531ec

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[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	954.4 ± 17.5	941.0	1000.1	1.00
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	960.2 ± 14.9	936.0	989.8	1.01 ± 0.02

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	78.6%	5053/6429
🟡	Branches	67.18%	2059/3065
🟡	Functions	73.16%	834/1140
🟡	Lines	78.88%	4767/6043

Test suite run success

836 tests passing in 120 suites.

Report generated by 🧪jest coverage report action from 0e531ec

[tatomyr]

Almost there. Left a couple of comments.

[tatomyr]

Thank you!