SourceLocation line_position is undocumented

[stefnotch]

Description
The source location line_position does not indicate whether it is in UTF-8 code units, or if it counts code points (~characters)

wgpu/naga/src/span.rs

Lines 116 to 117 in 6040820

	/// 1-based column of the start of this span
	pub line_position: u32,

Observed behavior
The actual computation counts characters

wgpu/naga/src/span.rs

Line 76 in 6040820

let line_position = source[line_start..self.start as usize].chars().count() as u32 + 1;

[stefnotch]

Documenting this should be valuable, since Firefox is a famous user of wgpu and even they messed it up https://bugzilla.mozilla.org/show_bug.cgi?id=1885238