Add tooltip documentation to web request, text, and brush nodes

[PolyglotOpenstreetmap]

• Add descriptive tooltips to web request nodes (GET/POST, string/image conversion, resource loading, image decoding)
• Add tooltip to GPU rasterization node explaining footprint usage
• Improve text node tooltip with clearer per_glyph_instances parameter explanation
• Add tooltips to brush stamp generator and blit operations explaining their internal roles
• Fix typos in tooltip and variable names (it's→its, perview→preview, instnace→instance)

Closes #

[TrueDoctor]

@PolyglotOpenstreetmap can you address the remaining comments?

[PolyglotOpenstreetmap]

I made a new commit, POST was commented with a message about GET.

[TrueDoctor]

Pinging @Keavon