Note that this feature is not available in latest released pasdoc version, 0.10.0. You must download and compile pasdoc yourself or use DevelopmentSnapshots to use this feature.
You can use @include tag to include text from another file into a description. For example:
1 { This is some procedure.
2 @include(some_procedure_large_docs.txt) }
3 procedure SomeProcedure;
Inside file some_procedure_large_docs.txt you can use all normal pasdoc features, @-tags etc. (see WritingDocumentation).
The @include tag may be handy when documentation for given item is so large that it's more comfortable to keep it in entirely separate file. Or when you want to include some documentation text more than once — because you can @include the same file as many times as you wish.