DOC: Update compiler guidance for Windows
... to make it a bit more encouraging.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24331?src=pr&el=h1) Report
> Merging [#24331](https://codecov.io/gh/pandas-dev/pandas/pull/24331?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/216986d4691297d5cfec33b5c62be7890b9a54d7?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24331/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24331?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24331      +/-   ##
==========================================
+ Coverage   92.28%   92.28%   +<.01%     
==========================================
  Files         162      162              
  Lines       51831    51831              
==========================================
+ Hits        47830    47831       +1     
+ Misses       4001     4000       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.68% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24331?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24331/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.58% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24331?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/24331?src=pr&el=footer). Last update [216986d...6a999af](https://codecov.io/gh/pandas-dev/pandas/pull/24331?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/24331?src=pr&el=h1) Report
> Merging [#24331](https://codecov.io/gh/pandas-dev/pandas/pull/24331?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/216986d4691297d5cfec33b5c62be7890b9a54d7?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/24331/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24331?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24331   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files         162      162           
  Lines       51831    51831           
=======================================
  Hits        47830    47830           
  Misses       4001     4001
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.68% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24331?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/24331?src=pr&el=footer). Last update [216986d...6a999af](https://codecov.io/gh/pandas-dev/pandas/pull/24331?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jamesmyatt 