claim about manipulating shader `uniform` though JS API is ambiguous

[westnordost]

The docs state

The uniforms block allows shortcuts for declaring globally-accessible uniform variables, for use in the global, position, normal, color and filter blocks. Uniforms declared here may also be accessed and manipulated through the JavaScript API.

Source: https://github.com/tangrams/tangram-docs/blob/44939085d5145417b2a5cd18e57287864bde33b7/docs/Syntax-Reference/shaders.md#uniforms

This can be misunderstood to mean that uniforms can be defined dynamically depending on certain feature properties. The documentation should make clear that a shader is in that sense static (or whatever is the best word).

Follow-up of tangrams/tangram-es#2319