Add documentation for OWNCLOUD_HTTP2_ENABLED variable

[AlexTurff]

☑️ Resolves

Missing (useful) env variable not being documented

Proof it exists:
https://help.nextcloud.com/t/nextcloud-client-and-http2-support/177698/3
https://help.nextcloud.com/t/windows-client-upload-awful-for-small-files/237634

🖼️ Screenshots

[joshtrichards]

Relevant code PR:

nextcloud/desktop#7182

I believe this change is fine, but tagged in a couple relevant possible reviewers on the Desktop client team to double-check.

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