[p5.strands] Document `getTexture()`

[davepagurek]

Increasing access

This is a central function in p5.strands and is referenced in e.g. baseFilterShader docs, but is not itself documented anywhere.

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

Should include its parameters and some usage examples.

[nbogie]

I'd like to take this one, please, @davepagurek