hover: use `***` to print dividers.

[voodoos]

vscode renderer treats the section before a --- divider as a title (it is a setext heading, see example 59). This stayed undetected before the syntax doc PR because only code was printed before the divider.

Current behavior with ---:

We can use stars to get a delimiter without a heading:

cc @PizieDust

[coveralls]

Pull Request Test Coverage Report for Build 4377

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 22.029%

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Totals
Change from base Build 4363:	0.0%
Covered Lines:	5425
Relevant Lines:	24627

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💛 - Coveralls