Decide on KDoc tags case-sensitivity

[qwwdfsad]

#2907 uncovered that our Javadoc's tag parser is case-insensitive even though it shouldn't be according to specification.

Further investigation has shown that KDoc tags are, as opposed to Javadoc's tags, case-insensitive: https://github.com/JetBrains/kotlin/blob/92d200e093c693b3c06e53a39e0b0973b84c7ec5/compiler/psi/src/org/jetbrains/kotlin/kdoc/parser/KDocKnownTag.kt#L42

We should decide whether this behaviour is deliberate or was driven by the implementation convenience, and react accordingly: change the behaviour or change the specification.