RFR: JDK-8276652: Missing row headers in MethodHandles.Lookup docs

[Ludvig Janiuk]

Added row headers missing in the table captioned "Access mode summary". This carries no visual change, but might be useful for screen readers.

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Commit messages:
 - Added table headers

Changes: https://git.openjdk.java.net/jdk/pull/6261/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6261&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276652
  Stats: 34 lines in 1 file changed: 0 ins; 0 del; 34 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6261.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6261/head:pull/6261

PR: https://git.openjdk.java.net/jdk/pull/6261