List limits for Limits::downlevel_defaults and downlevel_webgl2_defaults in docs.

[JustAnotherCodemonkey]

Checklist

• Run cargo clippy.
• Run cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Description
When looking at the docs for Limits, the actual limits returned by the above default functions are not listed. This PR, among a few other small changes to make things more clear, lists the values for the Limits returned by each of those functions within the docs. This PR makes the following changes:

1. Lists the values for Limits::downlevel_defaults and Limits::downlevel_webgl2_defaults in the documentation for Limits.
2. Changes the memory size units used the the docs for Limits to be their binary unit counterparts to avoid confusion (MiB instead of MB as a true megabyte is 1000 KB instead of 1024 even though it is often used to mean the latter, as is done in the docs as they stand).
3. Refactors the value of max_buffer_size used for both <Limits as Default>::default and downlevel_defaults from 1 << 28 to 256 << 20 for clarity and readability. This follows the pattern of both max_uniform_buffer_binding_size and max_storage_buffer_binding_size which use a similar notation.

[Wumpf]

Thanks for the improvements!

personally I'm not a huge fan of duplicating the constant values from the code into the docs (in particular those that aren't dicated by the spec), but I get that it's quite annoying when browsing e.g. docs.rs.

[cwfitzgerald]

To make sure these stay in sync, if you change the list to a doctest, you can assert the documented list is the same as that returned by the function.

[JustAnotherCodemonkey]

What do you mean? How do I make a doctest without changing how the docs look in the end result? I would love to do that.

[cwfitzgerald]

The documentation will end up looking different - it would be in a code block, and we'd need something other than bold to determine which ones have changed.

[JustAnotherCodemonkey]

I'll try that. I'll add another commit and see what you think.

[JustAnotherCodemonkey]

Ok so I marked those changed from defaults to downlevel defaults are marked with a * and those changed from downlevel defaults in downlevel webgl2 defaults are marked with a + with carryover *'s. I do worry about the accessibility of that it might be hard to distinguish. I've thought of some other symbol combinations but they don't make as much sense. What do you guys think?

[Wumpf]

nice! This is a much better solution!