RFR: JDK-8262875: doccheck: empty paragraphs, etc in java.base module

[Jonathan Gibbons]

Please review some minor doc fixes, for issues found by _doccheck_.    There are two kinds of errors that are addressed.

1. Incorrect use of `<p>`. In HTML, `<p>` marks the *beginning* of a paragraph. It is not a terminator, to mark the end of a paragraph, or a separator to mark the boundary between paragraphs.  In particular, it should not be used at the end of a description before a javadoc block tag, such as `@param` or before other HTML block tags, like `<ul>` or `<table>`.

2. References to the id `package-description`, following the recent standardization of all ids generated by javadoc,

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Commit messages:
 - JDK-8262875: doccheck: empty paragraphs, etc in java.base module

Changes: https://git.openjdk.java.net/jdk/pull/2795/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2795&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262875
  Stats: 9 lines in 8 files changed: 0 ins; 1 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2795.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2795/head:pull/2795

PR: https://git.openjdk.java.net/jdk/pull/2795