[jfx15] RFR: 8250799: NumberStringConverter and its subclasses are missing documentation for all their constructors

[Nir Lisker]

This was missed the in Javadoc fixes for 15.

Added documentation for all the constructors, did a bit of formatting, and removed redundant comments.

-------------

Commit messages:
 - Initial commit of 8250799

Changes: https://git.openjdk.java.net/jfx/pull/277/files
 Webrev: https://webrevs.openjdk.java.net/jfx/277/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8250799
  Stats: 67 lines in 3 files changed: 46 ins; 11 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx/pull/277.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/277/head:pull/277

PR: https://git.openjdk.java.net/jfx/pull/277