Fix MA0018 reporting one diagnostic per event accessor

[meziantou]

What changed

DoNotDeclareStaticMembersOnGenericTypes (MA0018) skipped PropertyGet and PropertySet accessors so that a static property is reported once, through its IPropertySymbol. The event accessor kinds were missing from that filter, so MethodKind.EventAdd and MethodKind.EventRemove are now skipped too. The comment, which said "skip properties" while the code skips accessors, was corrected.

Why

For a static event, GetMembers() returns the IEventSymbol and its add_ / remove_ accessors, all three public and static, so all three were reported:

public class Sample<T>
{
    public static event EventHandler MyEvent;
}

produced three identical diagnostics at the same location:

/0/Test0.cs(4,38): info MA0018: Do not declare static members on generic types
/0/Test0.cs(4,38): info MA0018: Do not declare static members on generic types
/0/Test0.cs(4,38): info MA0018: Do not declare static members on generic types

With explicitly written add / remove accessors the three diagnostics land on three different spans, so the IDE does not even deduplicate them.

Notes for reviewers

• Using member.IsImplicitlyDeclared instead would not be enough: accessors written explicitly are not implicit.
• The compiler-generated backing field of a field-like event is private, so IsVisibleOutsideOfAssembly() already excluded it — it never contributed a diagnostic.
• Two tests were added to DoNotDeclareStaticMembersOnGenericTypesTests: a field-like static event and one with explicit add / remove accessors (the case with distinct spans). Temporarily reverting the analyzer change makes exactly those two fail, so they do pin the defect.
• All 11 tests in the class pass on roslyn4.8, 4.14, 5.0, 5.6 and 5.9.
• dotnet run --project src/DocumentationGenerator exits 0 with no markdown changes. docs/Rules/MA0018.md needs no update: the documented behavior is unchanged, this only removes duplicates.