Docs: update description for pipeline.spec_template.revision (#115)

## What

* Update description for `pipeline.spec_template.revision`

## Why

* #74
(opened before documentation overhaul)

## Notes
<!-- Add any notes here -->

Closes
#74

## Checklist

* [x] _I have read
[CONTRIBUTING.md](https://github.com/codefresh-io/terraform-provider-codefresh/blob/master/README.md)._
* [x] _I have [allowed changes to my fork to be
made](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork)._
* [x] _I have added tests, assuming new tests are warranted_.
* [x] _I understand that the `/test` comment will be ignored by the CI
trigger [unless it is made by a repo admin or
collaborator](https://codefresh.io/docs/docs/pipelines/triggers/git-triggers/#support-for-building-pull-requests-from-forks)._

Author: korenyoni

codefresh/resource_pipeline.go

@@ -123,7 +123,7 @@ Or: <code>original_yaml_string = file("/path/to/my/codefresh.yml")</code>
 										Required:    true,
 									},
 									"revision": {
-										Description: "The git revision of the spec template.",
+										Description: "The git revision of the spec template. Possible values: '', *name of branch*. Use '' to autoselect a branch.",
 										Type:        schema.TypeString,
 										Required:    true,
 									},

docs/resources/pipeline.md

@@ -169,7 +169,7 @@ Required:
 
 - `path` (String) The relative path to the Codefresh pipeline file.
 - `repo` (String) The repository of the spec template (owner/repo).
-- `revision` (String) The git revision of the spec template.
+- `revision` (String) The git revision of the spec template. Possible values: '', *name of branch*. Use '' to autoselect a branch.
 
 Optional: