Document font structure layout constraints

This has to be the same as in src/ftcrfont.c and src/ftfont.c

* src/xftfont.c (struct xftfont_info): Document layout constraints.
1 job for emacs-26 in 5 minutes and 28 seconds
Status Job ID Name Coverage
failed #285


Name Stage Failure
test Test
20413152 of 33554432 static heap bytes used
/bin/bash: line 1: 10428 Segmentation fault (core dumped) ./temacs --batch --load loadup bootstrap
Makefile:739: recipe for target 'bootstrap-emacs' failed
make[1]: Leaving directory '/builds/tzz/emacs/src'
make[1]: *** [bootstrap-emacs] Error 139
Makefile:421: recipe for target 'src' failed
make: *** [src] Error 2
ERROR: Job failed: exit code 1