Add Javadoc documentation to Cache.compute method

[Tanush-Jain]

This pull request is addressing the issue of
"Cache's compute misreports the removal cause of a collected entry #7985"
so it adds comprehensive Javadoc documentation to the compute method in the Cache interface (guava/guava/src/com/google/common/cache/Cache.java). The method was previously undocumented, which was inconsistent with the detailed documentation provided for other methods in the interface.

Changes Made :

1. Added detailed Javadoc comment to the compute method, describing its atomic behavior, parameters, return value, and exception handling.
2. Included the @CanIgnoreReturnValue annotation for consistency with similar methods.
3. Added the @SInCE 23.0 annotation to indicate the version when this method was introduced.
4. Ensured the documentation style matches the existing Javadoc format in the file.

Testing:
No functional changes were made; this is purely a documentation update. Existing tests should continue to pass.

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