DOC: Clarify DST rounding behavior in Timestamp/DatetimeIndex
xref https://github.com/pandas-dev/pandas/issues/44287
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them

just wondering out loud if we could/should actually do this in a tz-aware way for `.round`, `.ceil`, `.floor`; e.g. are these fundamentally different ops that say subtraction that we could *always* do this w/o going ambiguous

cc @mroeschke  @jreback found discussion https://github.com/pandas-dev/pandas/pull/22560#issuecomment-419334705 and  https://github.com/pandas-dev/pandas/pull/22560#issuecomment-419684301 that rounding in a tz-aware might not actually be possible and falling back to rounding based on actual, UTC time may give unexpected results.
 my question is can we leave in thr tz and do a round operation w/o introducing ambiguity? Hmm since the rounding operations happen on epoch timestamps, rounding could happen on the UTC epoch timestamps and relocalized without ambiguity i.e. `.tz_convert("UTC").round(...).tz_convert(tz)`. But this flies counter to https://github.com/pandas-dev/pandas/pull/22560#issuecomment-419684301 and the notion of "round relative to the wall time" @mroeschke rereading i am wondering if it's always possible in freq < day (eg hour / min) to do the round ops in local time (and not convert-round-localize) which can introduce ambiguity Yeah for small rounding frequencies this can be achieved. Looking at the UTC offsets I think the limit is sub 15 minutes (I see a +05:45 UTC offset for example) thanks @mroeschke 