update regex for custom properties. update docs.

[greatestusername]

Resolves #39
Addresses #40

• Adds doc links to the API for source of truth on settings for steps, requests, transactions
• Addresses docs issue where country was shown as an allowed attribute on locations
• Fixes regexes for custom properties on checks to include _ and - characters

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Before the change?

• Custom properties would not accept as valid entries with _ or - characters

After the change?

• Custom properties now accept valid entries with _ or - characters

Pull request checklist

• Acceptance Tests have been updated, run (make testacc), and pasted in this PR (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Acceptance Test Output

terraform-provider-synthetics-fork 12:57> make testacc                                            
TF_ACC=1 go test $(go list ./... | grep -v 'vendor')  -timeout 120m   | sed '/X-Sf-Token/d'
?       github.com/splunk/terraform-provider-synthetics [no test files]
ok      github.com/splunk/terraform-provider-synthetics/synthetics      38.660s

Does this introduce a breaking change?

• Yes
• No

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[greatestusername-splunk]

LGTM
Just docs and regex validation changes