RFR: 8343819: Link Float.NaN and Double.NaN to equivalence discussion in Double

[Eirik Bjørsnøs]

Please review this doc-only enhancement which links the word _equivalent_ in `Float.NaN` and `Double.NaN` constant field descriptions to the floating-point equivalence discussion in `Double`.  

> It is equivalent to the value returned by{@code Float.intBitsToFloat(0x7fc00000)}.

For readers not well-versed in floating point, it may not be immediatly clear what _equivalent to_ means here.

The smallest improvement I could think of is to simply make the word `equivalent` a plain link to the floating-point equivalence discussion in the class description of `java.lang.Double`.

Verification: This is a doc-only change. I ran `make docs` and verified that the links resolve.

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Commit messages:
 - Link Double.NaN and Float.Nan to the floating-point equivalence discussion in Double##equivalenceRelation.

Changes: https://git.openjdk.org/jdk/pull/21972/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21972&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343819
  Stats: 6 lines in 2 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/21972.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21972/head:pull/21972

PR: https://git.openjdk.org/jdk/pull/21972