Add doc string context to Identifier validion regex rule

[QMalcolm]

Problem

The regex pattern for Identifier string validation had no context on WHY the restriction of identifiers was
what it was.

Solution

Add a docstring documenting the reason for the regex.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[codecov]

Codecov Report

Merging #8440 (806af2e) into main (ac539fd) will not change coverage.
Report is 3 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #8440   +/-   ##
=======================================
  Coverage   86.31%   86.31%           
=======================================
  Files         174      174           
  Lines       25561    25561           
=======================================
  Hits        22062    22062           
  Misses       3499     3499           

Flag	Coverage Δ
integration	83.09% <ø> (ø)
unit	65.14% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
core/dbt/contracts/util.py	92.36% <ø> (ø)