[foreign-memaccess+abi] RFR: Fix javadoc issues in MemorySegment::copy

[Maurizio Cimadamore]

This PR fixes few issues in the specification of `MemorySegment::copy`:

* in some code snippets `srcSegment` and `dstSegment` is followed by parenthesis (e.g. like a method call)
* the more general segment-to-segment `MemorySegment::copy` overload is missing a `throws` clause for when src segment is out of bounds
* the less general segment-to-segment `MemorySegment::copy` could use some splitting of its `throws` clauses, to make it more consistent with other methods

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/879/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=879&range=00
  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/879.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/879/head:pull/879

PR: https://git.openjdk.org/panama-foreign/pull/879