Docs: Improve branch/tagging branch fast-forward (branch always on HEAD) semantics

Context: https://github.com/trinodb/trino/pull/19111#discussion_r1334280732

Update the [Branching/Tagging page on the Iceberg docs](https://iceberg.apache.org/docs/latest/branching/) to explain how the [branching/tagging policies differ from what you may expect from git](https://github.com/apache/iceberg/pull/5364#issuecomment-1227902420).