DOC: Add infer frequency to docs

[jnmclarty]

Was there a reason infer_freq and inferred_freq weren't in the docs? Did I pick the right spot to add it? Does the feature work well?

I set out looking for a feature like this, searching "guess frequency", "auto detect frequency".

[jreback]

this is also defined for TimedeltaIndex

[jreback]

minor change, ping when pushed.

[jorisvandenbossche]

@jnmclarty Could you also update the infer_freq docstring with:

• it also works for TimedeltaIndex
• it raises a ValueError when not having 3 values? (now it only mentions a TypeError when the the provided values are no DatetimeIndex)

[jreback]

@jorisvandenbossche side issue, we don't appear to have PeriodIndex at all in api.rst....do we have an issue for this?

[jorisvandenbossche]

Ideally, we should also resolve this one: #5082

[jorisvandenbossche]

On the PeriodIndex API docs: yes, the issue is here: #6384

[jreback]

@jorisvandenbossche on #5082

In reality, I just think we could just fix infer_freq & infered_freq to return an actual offset object (which reprs to a string). rather than adding addtional methods. These compare correctly to strings anyhow.

[jnmclarty]

@jreback Okay to merge.

[jreback]

thanks!