doc: update documentation to point to security best practice

[cuiy0006]

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.91%. Comparing base (63bc533) to head (b9cc1e3).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14942   +/-   ##
=======================================
  Coverage   92.91%   92.91%           
=======================================
  Files        2351     2351           
  Lines      210082   210082           
=======================================
+ Hits       195202   195203    +1     
+ Misses      14880    14879    -1     

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[scotthart]

Reviewable status: 0 of 3 files reviewed, 2 unresolved discussions (waiting on @cuiy0006)

---

google/cloud/credentials.h line 122 at r2 (raw file):

 *   credentials.
 *
 * Important: If you accept a credential configuration (credential

s/Important:/@warning

This will add some additional highlighting during doc rendering.

Code quote:

Important:

---

google/cloud/credentials.h line 276 at r2 (raw file):

 * Azure, or SAML 2.0.
 *
 * Important: If you accept a credential configuration (credential

s/Important:/@warning

This will add some additional highlighting during doc rendering.

Code quote:

Important:

[scotthart]

Reviewed 2 of 3 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @cuiy0006)