fix code blocks in doc comments inconsistently using 3 or 4 spaces of indentation

[antonilol]

The metadata collector script was treating the space lines all start with as indentation. This caused code block's triple backticks to get a space in front of it, like this:

 ```rust
^ this space

Code after that that is indented with 4 spaces will only have 3 of those rendered.
Example (taken from here):

...
pub fn divide_by(x: i32, y: i32) -> i32 {
   if y == 0 {                      // 3 spaces
       panic!("Cannot divide by 0") // 7 spaces
...

Also added 'compile_fail' alongside the other rustdoc directives (second code block here had no highlighting), fixed a doc comment using 'rs' instead of 'rust' and removed some spaces causing an extra missing space of indentation (see second code block here).

changelog: none

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[xFrednet]

r? @xFrednet

[xFrednet]

Nice catch, thank you for the PR :D

cc: @Alexendoo this adds a slight modification to the metadata collector. You'll likely have to rebase your PR and copy the changes as well #13221

---

Roses are red,
•Violets are blue,
••Spaces are added,
•••Indention is cool,

[bors]

📌 Commit d7f1252 has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit d7f1252 with merge e070784...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing e070784 to master...