graphics/proj: further improve port installation message text

Follow up on commit 595b35fa1f9f and fix a few minor textual nits:

• Terminate `cf.' with the period as it is an abbreviation
• Avoid addressing the reader using `you/your' pronouns
• Use correct units (it refers to gigabytes, not gigabits)
• Expand contractions (dir -> directory), reduce tautology, wrap overly long lines, etc.