HTML5: document host-set Module opt-outs (defold/defold#12396 follow-up)

[birdingman0626]

Follow-up documentation for defold/defold#12396, which expands dmloader.js to respect six host-set properties on the global Module object:

• Module.isWASMPthreadSupported
• Module.isWebGL2Supported
• Module.webGLContextAttributes
• Module.webGLExtensionFilter
• Module.showButtonStrip
• Module.autoReloadOnWebGLContextRestore

Each is captured before the loader's own var Module = {...} redefinition and applied in an IIFE that runs at the same site. Default behaviour is byte-identical when nothing is pre-set.

This PR adds a new "Host-set Module properties" subsection to docs/en/manuals/html5.md, placed between the existing "Engine arguments" and "Query arguments in the URL" subsections under "Passing arguments to an HTML5 game". The new content includes:

• The example shape (a var Module = { ... }; block before the <script src=\"dmloader.js\"> tag).
• A per-property table with the type constraint, the effect, and the concrete failure mode each opt-out addresses.
• Two callouts: the let vs var binding gotcha (top-level let Module is invisible to the loader's var Module), and the strict sentinel-value semantics (=== false / === true / function typeof, no coercion).

Should land after defold/defold#12396 merges, since this references behaviour the engine doesn't have yet. Adjustable to whatever the final API surface looks like if the engine PR shrinks during review.

PR checklist

• Documentation
  • Docs change matches the new engine-side behaviour described in Allow WebView-host opt-outs via pre-set Module.* properties defold#12396.
  • New section inserted into the existing manual structure rather than a new file, so navigation / links stay consistent.
  • Markdown anchors and code fences validated locally.