New config option to support lower width limit on code blocks in doc comments

[calebcartwright]

The motivation for this new option is outlined in #5345, and the option proposed here is a potential solution to that scenario.

Specifically, we need to introduce a new config option named something like doc_comment_code_block_width (but feel free to come up with a better name) that's used to set the width threshold used when rustfmt is reformating code blocks residing within doc comments.

This should be a fairly straightforward option to add, and a great way to start getting familiar with rustfmt, for anyone that's interested!

Implementation pointers:

• Add an entry for the new option in the Configurations.md file, and the configuration mod, default value should also be 100
• Update the cloned config value that's used when formatting the code block here:
  

  rustfmt/src/comment.rs

  Lines 726 to 742 in 79515f1

  	let code_block = match self.code_block_attr.as_ref().unwrap() {
  	CodeBlockAttribute::Rust
  	if self.fmt.config.format_code_in_doc_comments()
  	&& !self.code_block_buffer.is_empty() =>
  	{
  	let mut config = self.fmt.config.clone();
  	config.set().wrap_comments(false);
  	if let Some(s) =
  	crate::format_code_block(&self.code_block_buffer, &config, false)
  	{
  	trim_custom_comment_prefix(&s.snippet)
  	} else {
  	trim_custom_comment_prefix(&self.code_block_buffer)
  	}
  	}
  	_ => trim_custom_comment_prefix(&self.code_block_buffer),
  	};

I suspect what you'll want to do is ensure the max_width value on the cloned config is overridden to the lowest value between the new doc_comment_code_block_width or max_width (since the overarching max_width needs to be the ceiling anyway)

#5228 may also serve as a helpful reference for adding new options