RFR: 8276338: Minor improve of wording for String.to(Lower|Upper)Case

[Pavel Rappo]

This PR fixes two sentences which conflate a string with its length, and also makes the "equivalency wording" consistent.

There are many ways to fix "the resulting String may be a different length than the original String". The proposed way might be one of the most lightweight ways to do that.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6232/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6232&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276338
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6232.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6232/head:pull/6232

PR: https://git.openjdk.java.net/jdk/pull/6232