docs: note how to use a phone as a camera in WebCam and VideoCamera

[petercorke]

Summary

VideoCamera already documents Continuity Camera (a Mac plus an iPhone shows up as a local camera). This adds the other half, prompted by looking at the old PhoneSensor experiment (a 2021 package that served a web page to stream camera and IMU data from a phone):

• WebCam passes its url straight to cv2.VideoCapture, so it accepts anything OpenCV can open: a still-image URL, an HTTP (MJPEG) stream or RTSP. A phone running an IP-camera app that publishes a stream can therefore be used by URL. New note in the WebCam docstring says so, and points to VideoCamera for Continuity Camera.
• VideoCamera note and :seealso: now point to WebCam for other phones and networked cameras.
• Fixes a typo ("Manu webcameras").

Docstring-only change; no code changed.

Verification, and what I did not do

• Imported the module and checked the docstrings contain the new text.
• Not tested with a real phone app: the wording says which stream types work "depends on the app and on how your OpenCV was built" for that reason.
• Docs not built locally; I followed the file's existing conventions (short :class: names resolved by the stubs' currentmodule, and a literal for cv2.VideoCapture because the docs run nitpicky). The CI build job builds the docs on this PR.

Checklist

• PR title follows Conventional Commits
• Docstrings updated
• CI green

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

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