Update doc of ColumnBuilder::text()

[Tigrov]

Related PR

• Add test for TEXT type with different sizes db-mysql#370

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	yiisoft/db-mysql#2

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.53%. Comparing base (5502bc2) to head (78d6711).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #910   +/-   ##
=========================================
  Coverage     99.53%   99.53%           
  Complexity     1550     1550           
=========================================
  Files            82       82           
  Lines          3834     3834           
=========================================
  Hits           3816     3816           
  Misses           18       18           

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