From type/Metamodel/Documenting

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method WHY()

Returns the documentation for a type.

From type/Mu

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multi method WHY(Mu: --> Pod::Block::Declarator)

Returns the attached Pod::Block::Declarator.

For instance:

#| Initiate a specified spell normally 
sub cast(Spell $s) {
  do-raw-magic($s);
}
#= (do not use for class 7 spells) 
say &cast.WHY;
# OUTPUT: «Initiate a specified spell normally␤(do not use for class 7 spells)␤» 

See Pod declarator blocks for details about attaching Pod to variables, classes, functions, methods, etc.