Add basic documentation for text entry box

[MyreMylar]

fixes #381

[codecov]

Codecov Report

Base: 97.57% // Head: 97.57% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (33f5f9f) compared to base (b218330).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #388   +/-   ##
=======================================
  Coverage   97.57%   97.57%           
=======================================
  Files          81       81           
  Lines       11214    11215    +1     
=======================================
+ Hits        10942    10943    +1     
  Misses        272      272           

Impacted Files	Coverage Δ
pygame_gui/elements/ui_text_box.py	99.44% <ø> (ø)
pygame_gui/elements/ui_text_entry_box.py	94.54% <100.00%> (+0.01%)	⬆️

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