Updating ByteBuf Javadocs to represent actual behaviour. (netty#8709)

Motivation:

The javadocs stating `IndexOutOfBoundsException` is thrown were
different from what `ByteBuf` actually did. We want to ensure the
Javadocs represent reality.

Modifications:

Updated javadocs on `write*`, `ensureWriteable`, `capacity`, and
`maxCapacity` methods.

Results:

Javadocs more closely match actual behaviour.