RFR: JDK-8274666: rename HtmlStyle.descfrmTypeLabel to be less cryptic

[Jonathan Gibbons]

Please review s simple update to replace use of the cryptic string "descfrm" with either "descriptionFrom" or "description-from" as appropriate.  

While mostly internal, the name does show up in the stylesheet.

Note: there's a test file called `custom-stylesheet.css` which still uses _camelCase_ naming even though the main stylesheet has been converted to _kebab-case_. While this does not really affect the test, we might want to (separately) clean up this naming mismatch.

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Commit messages:
 - JDK-8274666: rename HtmlStyle.descfrmTypeLabel to be less cryptic

Changes: https://git.openjdk.java.net/jdk/pull/5792/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5792&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274666
  Stats: 31 lines in 11 files changed: 0 ins; 1 del; 30 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5792.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5792/head:pull/5792

PR: https://git.openjdk.java.net/jdk/pull/5792