RFR: 8281223: Improve the API documentation of HttpRequest.Builder::build to state that the default implementation provided by the JDK returns immutable objects. [v2]

[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

Conor Cleary has updated the pull request incrementally with one additional commit since the last revision:

  8281223: Updated description, added missing code tag

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7479/files
  - new: https://git.openjdk.java.net/jdk/pull/7479/files/1bead0bd..bfc1de70

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7479&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7479&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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