Improve documentation w/ callouts for what compute will inherit admin permissions and what access admin permissions provide

**Describe the bug**

Improve documentation w/ callouts for what compute will inherit admin permissions, and 2) what access admin permissions provide
https://github.com/aws-amplify/amplify-category-api/issues/748

**To Reproduce**
Steps to reproduce the behavior:

1. Create a GraphQL API with cognito and IAM auth.
2. create Lambda function
3. update schema with @function and field level auth rule with group access.
4. create a function and provide resource access permissions to api -> query and mutation
5. add function code to call the query containing @function

**Expected behavior**
A clear and concise description of what you expected to happen.

**Screenshots**
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**Desktop (please complete the following information):**
- OS: [e.g. iOS]
- Browser [e.g. chrome, safari]
- Version [e.g. 22]

**Smartphone (please complete the following information):**
- Device: [e.g. iPhone6]
- OS: [e.g. iOS8.1]
- Browser [e.g. stock browser, safari]
- Version [e.g. 22]

**Additional context**
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