Lost docstrings in 21.5

[Aidan Kehoe]

 Ar an ceathrú lá de mí na Nollaig, scríobh Stephen J. Turnbull: 

 > Aidan, looks like you touched this stuff last:
 > 
 > Finding pointers to doc strings...
 > Finding pointers to doc strings...done
 > Warning: doc lost for function unicode-error-translate-region.
 > Warning: doc lost for function frob-unicode-errors-region.
 > Warning: doc lost for function add-menu-item.
 > Warning: doc lost for function add-menu.
 >
 > I don't see what the problem is, anybody know?

The problem is that all four functions (and a few more defvars that the
warning isn’t picking up) are within (when (featurep FEXP) ...) clauses.
This means that when make-docfile.c looks through the ELC code, its
generally stupid matching sees a (byte-code ...) for the entire form instead
of separate #'defalias and #'defvar forms for each function and variable. 

One way to eliminate the error would be to move the featurep check at
runtime, and throw an 'unimplemented error if it’s not available.

-- 
¿Dónde estará ahora mi sobrino Yoghurtu Nghé, que tuvo que huir
precipitadamente de la aldea por culpa de la escasez de rinocerontes?