docs(span): finish docs and tests for Span

[joshka]

[codecov]

Codecov Report

Merging #427 (1097f3d) into main (80fd77e) will increase coverage by 0.38%.
Report is 3 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
+ Coverage   87.08%   87.46%   +0.38%     
==========================================
  Files          40       40              
  Lines        9822    10139     +317     
==========================================
+ Hits         8553     8868     +315     
- Misses       1269     1271       +2     

Files Changed	Coverage Δ
src/text/span.rs	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

[mindoodoo]

Couple nitpicks otherwise LGTM ! Thanks for this PR, was needed !

[joshka]

Updated wording to simplify it somewhat. My reason for being more verbose than necessary is to make it easy on new rust developers (who may not have seen used Cow), and older tui-rs developers that may not be aware of the Stylize trait and the shortcut methods.

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