RFR: 8252882: Clean up jdk.javadoc and the related parts of jdk.compiler [v2]

[Pavel Rappo]

> Yet another cleanup change that has accumulated during my work on `DocCommentParser` and other javadoc-related areas of
> compiler.

Pavel Rappo has updated the pull request incrementally with one additional commit since the last revision:

  To address pull-request feedback, I removed 4 `assert` statements I introduced earlier.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/148/files
  - new: https://git.openjdk.java.net/jdk/pull/148/files/acb118a3..67fa917b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=148&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=148&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/148.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/148/head:pull/148

PR: https://git.openjdk.java.net/jdk/pull/148