Exclude code choosers in SDKs

[iwahbe]

Right now, we generate code blocks for the schema that look like this:

<!--Start PulumiCodeChooser -->
```typescript
// TS Code
```
```python
# Python Code
```
...
<!--End PulumiCodeChooser -->

That works great in the registry. For SDKs, we generate code that looks like this:

  # <!--Start PulumiCodeChooser -->
  # ```python
  # # Python Code
  # ```
  # <!--End PulumiCodeChooser -->

This is not good, since our users see our PulumiCodeChooser directives directly.

This PR changes the bridge's behavior to only emit the PulumiCodeChooser directives when we are targeting the schema.

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 60.01%. Comparing base (4dbf4df) to head (6eed2fe).

Files	Patch %	Lines
pkg/tfgen/docs.go	72.72%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1887   +/-   ##
=======================================
  Coverage   60.00%   60.01%           
=======================================
  Files         327      327           
  Lines       43986    43994    +8     
=======================================
+ Hits        26393    26401    +8     
  Misses      16101    16101           
  Partials     1492     1492           

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[guineveresaenger]

LGTM! Thanks for this improvement.