DOC: added string processing comparison with R
#13229

Added string processing section to comparison_with_r DOC. Used info from http://blog.dominodatalab.com/using-r-and-python-for-common-sas-functions.

This completes the issue.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=h1) Report
> Merging [#16502](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e81f3cc30725443a1d36ec27279f3fec4eed60ec?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16502/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16502   +/-   ##
=======================================
  Coverage   90.43%   90.43%           
=======================================
  Files         161      161           
  Lines       51045    51045           
=======================================
  Hits        46161    46161           
  Misses       4884     4884
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.27% <ø> (ø)` | :arrow_up: |
| #single | `40.16% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=footer). Last update [e81f3cc...bf8ae7a](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=h1) Report
> Merging [#16502](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e81f3cc30725443a1d36ec27279f3fec4eed60ec?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16502/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16502   +/-   ##
=======================================
  Coverage   90.43%   90.43%           
=======================================
  Files         161      161           
  Lines       51045    51045           
=======================================
  Hits        46161    46161           
  Misses       4884     4884
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.27% <ø> (ø)` | :arrow_up: |
| #single | `40.16% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=footer). Last update [e81f3cc...bf8ae7a](https://codecov.io/gh/pandas-dev/pandas/pull/16502?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung can you review can you rebase / update can you update according to comments closing as stale. if you'd like to continue working, pls ping.