wordgraph_pathpos_add(): Minor fixes.
wordgraph_pathpos_add(): Minor fixes.

1. Remove wordgraph_hier_position(p).
   It is now done at the start of flatten_wordgraph(), so we can assume
   that the hier_depth is valid here.
2. Improve/add comments.

ampli/link-grammar,linas/link-grammar,linas/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar,MadBomber/link-grammar,MadBomber/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,MadBomber/link-grammar,MadBomber/link-grammar,linas/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,MadBomber/link-grammar,MadBomber/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,ampli/link-grammar,MadBomber/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,MadBomber/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar