Add new potential annotations and update existing ones

[netdpb]

• Add METHOD and CONSTRUCTOR targets to @NullMarked (Allow annotating null-markedness at method/constructor level? #43).
• Update basic Javadoc for @NullMarked and @Nullable.
• Add @NullUnmarked (Annotation to negate @NullMarked at a narrower scope #127).
• Add @NonNull (Existence of @NonNull #230).
• Add @Implies (Declaring that one annotation aliases or specializes another (@Implies) [working decision: no] #35) (and its containing annotation @Implies.Container).