Doc/gitsubmodules: make some changes to improve readability and syntax

* Only mention porcelain commands in examples

* Split a sentence for better readability

* Add missing apostrophes

* Clearly specify the advantages of using submodules

* Avoid abbreviations

* Use "Git" consistently

* Improve readability of certain lines

* Clarify when a submodule is considered active

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>