docs: Improve readability of headings for functions.

[MarkDuckworth]

Fixes b/316190402

One change to the Functions table at the top of the page:

1. In function table, change function(param...) to function(param, ...)

Three changes to the function definitions at the bottom of the page:

1. When function definitions are sorted by first param, add section headers within function definitions to show the logical groupings of functions.
2. Add params to the function definition title, so readers can differentiate two adjacent headers
3. Fix the heading level of the Exception node.

Function definitions was:

## foo()
### params: app, Field
## Exception If something something

## foo()
### params: app, string
## Exception If something something

## bar()
### params: app, ...

## baz()
### params: app

## foo()
### params: firestore, ...

Function definitions is now:

## function(app, ...)

### foo(app, Field)
#### params: app, Field
#### Exception If something something

### foo(app, string)
#### params: app, string
#### Exception If something something

### bar(app, ...)
#### params: app, ...

### baz(app)
#### params: app

## function(firestore, ...)

### foo(firestore)
#### params: firestore, ...

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Latest commit: 6300032

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (4293fff) and merge commit (918172b).

Test Logs

• Base (4293fff): https://github.com/firebase/firebase-js-sdk/actions/runs/7186971319
• Merge (918172b): https://github.com/firebase/firebase-js-sdk/actions/runs/7198496278

1. https://storage.googleapis.com/firebase-sdk-metric-reports/KaBKyir0Hq.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (4293fff) and merge commit (918172b).

Test Logs

• Base (4293fff): https://github.com/firebase/firebase-js-sdk/actions/runs/7186971319
• Merge (918172b): https://github.com/firebase/firebase-js-sdk/actions/runs/7198496278

1. https://storage.googleapis.com/firebase-sdk-metric-reports/R9VBk6nIZw.html