docs: Add a note about expected newlines in multiline markdown fields

[lornajane]

What/Why/How?

Fixes #649 by adding a clarifying note that markdown fields may have trailing newlines.

Check yourself

• Code is linted
• Tested with redoc/reference-docs/workflows (internal)
• All new/updated code is covered with tests

Security

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

[changeset-bot]

⚠️ No Changeset found

Latest commit: dd5b617

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[github-actions]

Command	Mean [s]	Min [s]	Max [s]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	1.050 ± 0.022	1.024	1.101	1.00
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	1.051 ± 0.021	1.022	1.098	1.00 ± 0.03

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	76.31%	4100/5373
🟡	Branches	66.29%	2183/3293
🟡	Functions	68.42%	663/969
🟡	Lines	76.51%	3847/5028

Test suite run success

653 tests passing in 94 suites.

Report generated by 🧪jest coverage report action from dd5b617