`SuppressWarningsWithoutExplanation` Inconsistency Between Documentation and Implementation

There is an inconsistency between the documentation and the actual implementation of the `SuppressWarningsWithoutExplanation` bug checker in Error Prone.

### Description

1. The main documentation at https://errorprone.info/bugpattern/SuppressWarningsWithoutExplanation states the following:

- In the "**Introduction / One line summary**" section: "Use of `@SuppressWarnings` should be accompanied by a comment describing why the warning is safe to ignore."
- This implies **ALL** `@SuppressWarnings`
- In the "**The problem**" section: "Suppressions for `unchecked` or `rawtypes` warnings should have an accompanying comment..."
- This implies this check is only applicable to `@SuppressWarnings` for `unchecked` and `rawtypes`

2. The actual code implementation at [SuppressWarningsWithoutExplanation.java](https://github.com/google/error-prone/blob/master/core/src/main/java/com/google/errorprone/bugpatterns/SuppressWarningsWithoutExplanation.java):
- Code comment states: "The Google style guide mandates this for _all_ suppressions; this is only matching on `{@code deprecation}` as a trial."
- Implementation only checks for "deprecation" suppressions as shown in:
```Java
private static final Matcher SUPPRESS_WARNINGS =
allOf(
isSameType(SuppressWarnings.class),
hasArgumentWithValue("value", stringLiteral("deprecation")));
```

### Expected Behavior

Based on the Google style guide referenced in the code (which mandates explanatory comments for all suppressions) and the general documentation, this check should apply to ALL `@SuppressWarnings` annotations, not just those with "deprecation" value.
- Note: I cannot find a reference to this rule in the current published Java style guide at https://google.github.io/styleguide/javaguide.html

### Suggested Fix
1. Update the implementation to check all `@SuppressWarnings` annotations
2. Or update the implementation to check only `@SuppressWarnings` for `unchecked` or `rawtypes`
3. Or update the documentation to clearly state that this check currently only applies to "deprecation" suppressions