RFR: JDK-8279861: Clarify 'rect' parameters and description of paintTabBorder method in BasicTabbedPaneUI

[Alexey Ivanov]

The protected methods `paintTab` and `paintFocusIndicator` accept the `rect` parameter which is documented as _“rectangles”_. I suggest updating this description to a more descriptive: _“the tab rectangles”_ as the array contains the coordinates of the tabs.

Similarly, the documentation for `assureRectsCreated` method could be updated to clarify which rectangles are created.

The documentation for `paintTabBorder` is inconsistent with other paint methods, it has no ending punctuation. I updated it to be consistent.

I split out these changes from [JDK-8279798](https://bugs.openjdk.java.net/browse/JDK-8279798) and #7031 because this issue will likely require a CSR.

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Commit messages:
 - JDK-8279861: Clarify 'rect' parameters and description of paintTabBorder method in BasicTabbedPaneUI

Changes: https://git.openjdk.java.net/jdk/pull/7033/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7033&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279861
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7033.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7033/head:pull/7033

PR: https://git.openjdk.java.net/jdk/pull/7033