Documentation: Update Content API readme.

* Improved formatting and linking.
* Removed "Goal" which seems redundant now.
* Edited guidelines around enums and enum classes.

Change-Id: I11d922cbe310d93b9648aa0ff64fb7f99c1d1b1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824730
Reviewed-by: John Abd-El-Malek <[email protected]>
Commit-Queue: Matt Falkenhagen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#702261}
1 file changed