Document p5.strands instanceID() function

Increasing access

This currently has no docs but is useful for drawing many copies of something, and will also be an important step in using compute shaders.

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• WebGL
• Build process
• Unit testing
• Internationalization
• Friendly errors
• Other (specify if possible)

Feature enhancement details

See for example this sketch: https://openprocessing.org/sketch/2845379

This should also probably include a new example in the model docs, as you need to add a count to model(geom, n) to make everything work. Since this is an advanced usage of model, we should probably switch to inline examples in its documentation so that we can add instancing explanations after the more basic usage and before instancing examples.