Get extra nested docs from schema map

[guineveresaenger]

This pull request aims to improve accuracy for nested property docs descriptions.

It uses the SchemaMap to look up and read Description fields such as this one in the case where trying to obtain documentation form entityDocs does not yield any results.

The original impetus for this change was to address #1045, which is very specific to Config() fields. The Provider configuration docs are special-cased in the bridge and we do not have access to any entityDocs. Instead, we were reading in the TF schema's Description() field into rawdocs when generating a top-level config variable. This did not persist into any nested fields on Configuration variables, hence this fix. But it turns out that we can catch quite a few more missing descriptions if we extend implementation across all nested fields (see stats below).

As a side note - the Terraform Plugin Framework implements Description() in a way that respects and prioritizes descriptions that come from Markdown docs; for sdkv2 resources we still need to use, and prefer, our parsed-from-markdown entityDocs descriptions. It would be interesting to see if we get improved docs for TFPF resources by not reading descriptions from entityDocs.
It was also necessary to return an empty string from TFPF typeSchema so as not to cause a panic during schemagen on TFPF using providers. Open to improvements/suggestions here!

This PR aims to be fully additive - we are not changing the way in which docs are generated; we only fill in missing docs.
Nevertheless, this PR does also seem to improve the correctness of nested docs where some description fields have been matched to the wrong input field.

Please see a new Docker schema generated against these changes
Quick stats:

pulumi/pulumi-docker🦉 git diff master --stat -- provider/cmd
 provider/cmd/pulumi-resource-docker/schema.json | 111 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------------------------
 1 file changed, 68 insertions(+), 43 deletions(-)

Here is an AWS schema - you will want to pull that branch and look at the git diff to master locally.
Quick stats:

pulumi/pulumi-aws🦉 git diff master --stat -- provider/cmd 
 provider/cmd/pulumi-resource-aws/schema.json | 1810 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------------------
 1 file changed, 1206 insertions(+), 604 deletions(-)

This PR fixes #1045.

[iwahbe]

I can see the docker diff in GH: pulumi/pulumi-docker@master...nested-configs-sample-schema.

I believe that the corrections come from (downstream from the change):

pulumi-terraform-bridge/pkg/tfgen/generate_schema.go

Lines 567 to 573 in 6dc3163

	func (g *schemaGenerator) genProperty(prop *variable) pschema.PropertySpec {
	description := ""
	if prop.doc != "" && prop.doc != elidedDocComment {
	description = g.genDocComment(prop.doc)
	} else if prop.rawdoc != "" {
	description = g.genRawDocComment(prop.rawdoc)
	}

[iwahbe]

This is a big improvement, and I'm looking forward to getting this merged. I think we need to address one additional category of errors before merging.

[t0yv0]

Nit: "s" not needed.

[iwahbe]

A couple minuscule formatting nits. Otherwise LGTM!

[iwahbe]

Nit: There is a tfbridge.MakeResource(pkg, mod, res) function which does this already.

[iwahbe]

Suggested change

// MaxItems: 1,

[iwahbe]

Suggested change