RFR: 8281223: Improve the API documentation of HttpRequest.Builder::build to state that the default implementation provided by the JDK returns immutable objects.

[Conor Cleary]

As described in the title, this is a simple change to the `HttpRequest.Builder::build` method to highlight that an immutable and reusable instance of an `HttpRequest` is created when this method is invoked. This is done by adding an `@implSpec` Javadoc Tag (details on `@implSpec` given [here](https://openjdk.java.net/jeps/8068562)). 

The detail added under the `@implSpec` Tag is similar to that provided at the interface-level Javadoc for Builder.

Please also review the CSR for this change: https://bugs.openjdk.java.net/browse/JDK-8281833

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Commit messages:
 - 8281223: Improve the API documentation of HttpRequest.Builder::build to state that the default implementation provided by the JDK returns immutable objects.

Changes: https://git.openjdk.java.net/jdk/pull/7479/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7479&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281223
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7479.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7479/head:pull/7479

PR: https://git.openjdk.java.net/jdk/pull/7479