Remove redundant arrow in function literal without parameters / fix documentation

[paul-dingemans]

Description

Remove redundant arrow in function literal without parameters

Kotlin allows a function literal to be defined with an empty parameter list and a redundant arrow symbol to divide the parameter list from the body.

Closes #2331

Fix documentation as function-literal rule is experimental and not bound to ktlint_official code style.

Checklist

Before submitting the PR, please check following (checks which are not relevant may be ignored):

• Commit message are well written. In addition to a short title, the commit message also explain why a change is made.
• At least one commit message contains a reference Closes #<xxx> or Fixes #<xxx> (replace<xxx> with issue number)
• Tests are added
• KtLint format has been applied on source code itself and violations are fixed
• CHANGELOG.md is updated
• PR description added

Documentation is updated. See difference between snapshot and release documentation

• Snapshot documentation in case documentation is to be released together with a code change
• Release documentation in case documentation is related to a released version of ktlint and has to be published as soon as the change is merged to master
• In case of adding a new rule, it needs to be added to experimental rules documentation