generate-documentation failure on context mods

[Dan Eble]

I cannot tell why adding this to ly/context-mods-init.ly makes my build fail.

```
DoSomething = \with {
  \description "Blah blah @code{segnoBarType} blah blah @code{\\section}."
  segnoBarType = "S-||"
}
```

This is the end of Documentation/out/en/generate-documentation.log:

/home/user/lilypond-build/out/share/lilypond/current/scm/lily/document-context-mods.scm/home/user/lilypond-build/out/share/lilypond/current/scm/lily/lily.scmBacktrace:
          18 (primitive-load-path "lily/documentation-generate.scm")
In srfi/srfi-1.scm:
    640:9 17 (for-each #<procedure ly:load (a)> _)
In ice-9/eval.scm:
    619:8 16 (_ #(#(#(#(#<directory (lily) ffff8d0d5780>) "d?") #) #))
In unknown file:
          15 (primitive-load-path "lily/document-context-mods")
In ice-9/eval.scm:
   626:19 14 (_ #<module (#{ g210}#) ffff8e93ed20>)
   173:55 13 (_ #<module (#{ g210}#) ffff8e93ed20>)
    155:9 12 (_ _)
In srfi/srfi-1.scm:
   697:23 11 (filter-map #<procedure document-mod (a)> _ . _)
In ice-9/eval.scm:
   174:20 10 (_ #(#(#(#(#(#<module (#{ g210}#) ffff8e93?>) ?) ?) ?) ?))
   177:49  9 (lp (#<procedure ffff8f36be20 at ice-9/eval.scm:125:?> ?))
   177:49  8 (lp (#<procedure ffff8f36be00 at ice-9/eval.scm:273:?> ?))
   177:32  7 (lp (#<procedure ffff8f36bde0 at ice-9/eval.scm:182:7 ?>))
In srfi/srfi-1.scm:
   592:29  6 (map1 ((description "Blah blah @code{segnoBarType}?") #))
   592:17  5 (map1 ((assign segnoBarType "S-||")))
In ice-9/eval.scm:
    159:9  4 (_ #(#(#(#(#<module (#{ g210}#) ffff8e93ed20>) #) # ?) ?))
   245:16  3 (_ #(#(#(#(#<module (#{ g210}#) ffff8e93ed20>) #) # ?) ?))
    155:9  2 (_ #(#(#(#(#<module (#{ g210}#) ffff8e93ed20>) #) # ?) ?))
   223:20  1 (proc #(#(#(#(#<module (#{ g210}#) ffff8e93ed?>) ?) ?) ?))
In unknown file:
           0 (%resolve-variable (7 . value) #<module (#{ g210}#) fff?>)

ERROR: In procedure %resolve-variable:
Unbound variable: value

— 
Dan