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]

On Tue, 15 Feb 2022 15:33:01 GMT, Conor Cleary <ccleary at openjdk.org> wrote:

>> 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

Awaiting CSR approval and additonal review, keeping this PR active by commenting

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