committers-guide: add a bit about partial cloning

[pauamma_gundo.com]

For the next round, please include the whole file(s) as explained in https://wiki.freebsd.org/Phabricator#Create_a_Revision_via_Web_Interface. This lets reviewers make edit suggestions directly instead of laboriously explaining them.

documentation/content/en/articles/committers-guide/_index.adoc

478

s/If you are/If/ per FDP primer style guide.

480

s/you ask/asking/
s/you don't/it doesn't/

481

"Downloading happens under the hood, thus the command takes a bit longer to run."

485

s/kind/kinds/

488

s/listing, it contains/listing containing/
s/directory, and/directory and/ (no comma for 2-item lists)
s/sub directories/subdirectories/

489

"and pointers to the root directory tree and to the parent commit(s)" if I understand what you mean correctly (lowest directory in the tree below which all modified files are, reference to the single commit if not a merge, or to both or however many commits if a merge).

491

"the kind, number, and or size of objects downloaded"

500

"the branch requested"