Issue #2762173 by megansanicki, artinst4, YesCT, mradcliffe, a_thakur, vermauv, cilefen, mathieso, pwolanin, mlhess: Add "composer install" step to install.txt file for when Drupal is downloaded using git

(cherry picked from commit 4dcb2b1e)

Issue #2349565 by tengoku, jhodgdon, jribeiro, susanb, batigolix, no_angel, megansanicki: Review/update block_content hook_help text