--feature-flags command line arg is not documented

[redbaron]

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Is your feature request related to a problem? Please describe.
I was trying to disable manifest mode when invoking vcpkg in a dir with vcpkg.json and wasn't able to find any mentions of --feature-flags switch.

Setting VCPKG_FEATURE_FLAGS= didn't help either (I assume vcpkg treats present, but empty value as not present and defaults to builtin features).

I had to dig into source code to find --feature-flags=-manifests option

Proposed solution

1. add --feature-flags= to the --help output
2. document --feature-flags on vcpkg.io website
3. document how to disable feature with VCPKG_FEATURE_FLAGS env var

[github-actions]

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