RFR: 8334107: Specification for MemorySegment::get/setString could use some clarification

[Per Minborg]

This PR proposes to add additional text for the `@param charset` tags in the `MemorySegment.(get|set)String` overloads that take a `charset`. 

This PR does not need a CSR, as the specs already state that an exception will be thrown if the provided `charset` is not a standard charset.

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Commit messages:
 - Add param text to string operations in MemorySegment

Changes: https://git.openjdk.org/jdk/pull/21927/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21927&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334107
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21927.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21927/head:pull/21927

PR: https://git.openjdk.org/jdk/pull/21927