Move RelationshipType naming convention to Markdown guide instead

[bact]

• Put info about directionality and temporality to Relationship description.
• Move the sentences: "Relationship names should be descriptive enough to easily deduce the correct direction from their name. The best way to do this is to make sure that the relationship name completes the sentence.." out of the RelationshipType description and put them to the "Naming convention" section in the Markdown guide (docs/format.md) instead.
  • They are not meant for the users of the spec but for the authors of the spec.
• Update an example for property naming convention
  • Replace "import"/"imports" example with "element"/"elements", as "import"/"imports" can be both noun and verb and that can be confused with the relationship type naming convention.