Document new ImportC pragma for setting attributes

The new pragma is implemented in dlang/dmd#16820.
The documentation is based on the changelog entry.

spec/importc.dd

@@ -333,6 +333,33 @@ $(H2 $(LNAME2 preprocessor-directives, Preprocessor Directives))
  $(LI $(TT #pragma pack ( pop PopList )))
  )
 
+ $(H4 $(LNAME2 pragma-attribute, pragma attribute))
+
+ The following pragma for ImportC allows to set default storage
+ classes for function declarations:
+ ---
+ #pragma attribute(push, [storage classes...])
+ ---
+ The storage classes `nothrow`, `nogc` and `pure` are supported.
+ Unrecognized attributes are ignored.
+ Enabling a default storage class affects all function declarations
+ after the pragma until it is disabled with another pragma.
+ Declarations in includes are also affected. The following example
+ enables `@nogc` and `nothrow` for a library:
+
+ ---
+ #pragma attribute(push, nogc, nothrow)
+ #include <somelibrary.h>
+ ---
+
+ The changed storage classes are pushed on a stack. The last change can
+ be undone with the following pragma:
+ ---
+ #pragma attribute(pop)
+ ---
+ This can also disable multiple default storage classes at the same time,
+ if they were enabled with a single `#pragma attribute(push, ...)` directive.
+
 $(H2 $(LNAME2 _builtins, $(TT src/__builtins.di)))
 
  $(P The first thing the compiler does when preprocessing is complete is to import