[camera_platform_interface] Add web-relevant docs

[ditman]

This PR is the platform_interface part of: #4240.

git cherry-pick 52b8608f61f230ba89ef0e4acbfee0e0f6cce47b
git cherry-pick ad5057472e07df63604489ec344c6a47d2de02a5

It adds information relevant to the web to the platform interface docs.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

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[ditman]

@bselwe do you mind approving this PR? It's asking for your permission because I cherry-picked your commit into a branch on my fork, you just need to comment @googlebot I consent.

@mvanbeusekom / @BeMacized do you mind reviewing+landing this? It's a very simple change in the platform_interface docs. In case of any changes required, I think you guys have more "office time" overlap with @bselwe than I do :)

Once this is landed, I'll be able to rebase #4240 one final time, so only the camera example changes need to land!

Thanks everybody!

[mvanbeusekom]

LGTM, once @bselwe gives his consent I will make sure this will land.

[bselwe]

LGTM! Could we also cherry-pick this commit to include:

/// 2160p (3840x2160 on Android and iOS, 4096x2160 on Web)

[bselwe]

@googlebot I consent.

[ditman]

LGTM! Could we also cherry-pick this commit to include:

Arrrrgh, I didn't notice that one! I'll cherry-pick and merge! Thanks @mvanbeusekom!

[mvanbeusekom]

Anytime @ditman