Java: Improve "Learning CodeQL" docs

# Description
The "Learning CodeQL" / "CodeQL for Java" docs could probably be improved.

**Important:** When changing any queries, make sure to update the respective query console link as well.

## `types-class-hierarchy.rst`
- Could probably use `NullType` instead of checking for name `""`
- Also ignore `Wildcard` because it causes some false positives, or check that upper bound is same type or supertype?

## `annotations.rst`
- `@SuppressWarnings` `value` is an array so not necessarily a Literal, should therefore probably use `Annotation.getAValue(string)`

## `call-graph.rst`
- Says that for singleton pattern private constructor is not being called, however this is incorrect because constructor is called to create the singleton instance; it probably means "utility classes"

## `expressions-statements.rst`
- Why extend `ComparisonExpr` and override `getGreaterOperand` and `getLesserOperand` instead of using them? Would then not require two separate subclasses

## `introduce-libraries-java.rst`
- Might not check method body correctly, it appears there are statments which are the child of a method, but not its body, see [this query](https://lgtm.com/query/4799496553177136320/). However, maybe that it a bug with the extractor, it appears some of the results are for files which exist multiple times for the projects.