Document SQL comment hint functionality for custom types

[alcaeus]

The custom mapping type documentation does not mention the Type::requiresSQLCommentHint method. A recent change in DoctrineBundle 2.0 removes implicitly marking custom types as commented, instead deferring to the logic that is built into the type itself. However, since this isn't documented in the custom mapping types documentation, many people don't know about this and wonder why their custom types don't work.

For users that rely on implicit commenting in DoctrineBundle we introduced a deprecation layer in 1.11 that tells them to update their type, but users creating new types won't know about this. (see also doctrine/DoctrineBundle#1074 and doctrine/DoctrineBundle#977).

[morozov]

Irrelevant as of #5512.

[github-actions]

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