DOC: Add reference to frequency strings
- [x] closes #13160 

lots of places where this can be added.

asfreq, resample, Period, do a search. Once we decide on the text you can update.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13632?src=pr) is **84.38%**

> Merging [#13632](https://codecov.io/gh/pydata/pandas/pull/13632?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.04%**

``` diff
@@             master     #13632   diff @@
==========================================
  Files           138        142     +4   
  Lines         51095      51223   +128   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43093      43224   +131   
+ Misses         8002       7999     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [7c357d2...b45ae21](https://codecov.io/gh/pydata/pandas/compare/7c357d20f6cd0f379790c200e91075a179ebab75...b45ae21a354d0f9c90af787e84c44b09a64b8715?src=pr)
 @jreback please confirm the text once so that I can find and add the same at other places as well.
 @jorisvandenbossche lgtm.
 @sahildua2305 Thanks a lot!
