message: Add some doc to Mesh::FaceInfo.
diff: @@ -113,7 +113,8 @@ protected:
  Non-conforming slave faces always have the slave element as Elem1 and
  the master element as Elem2. On the other side, non-conforming master
  faces always have the master element as Elem1, and one of the slave
  - element as Elem2.
  + element as Elem2. Except for ghost non-conforming slave faces, where
  + Elem1 is the master side and Elem2 is the slave side.
  The indices of Elem1 and Elem2 can be indirectly extracted from
  FaceInfo::Elem1No and FaceInfo::Elem2No, read the note below for special
  @@ -158,7 +159,8 @@ protected:
  Their number cannot be known without counting them, indeed some ghost
  faces are included in the non-ghost faces... and most of the indices
  between `GetNumFaces()` and `faces_info.Size()` are unused. So counting
  - is mandatory.
  + is mandatory. -> Wrong shared non-conforming faces are not necessarily
  + ghost... (ghost just means f >= GetNumFaces()...)
  - Most of the ghost faces only countains -1 in all their attributes
  (that seems to be explained by pncmesh.cpp:1073-1074).
  The technical documentation below wonders if these are master
  