Use of possibly undefined variables in Context Processing

[kasei]

(Related to #265, #356)

Context Processing step 5.2.5 uses loaded context which is only defined in 5.2.4.3 (which is not necessarily evaluated prior to 5.2.5)

Context Processing step 5.2.5 uses context document which is only defined in 5.2.4 (which is not necessarily evaluated prior to 5.2.5)

[gkellogg]

Updated in #377.

[kasei]

@gkellogg new language in #377 looks good for this. Thanks.