doc: update readline asyncIterator docs

This commit:

• Removes an unnecessary stability index entry. These generally are not included for Stable entries.
• Reformat for-await-of.
• Remove mention of experimental status that is not true anymore.
• Remove use of "we"

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines