Replaced 'break' with 'beginning' for page|line|column|gathering in Guidelines and Specs

[trishaoconnor]

Replaced 'break' with 'beginning' for page|line|column|gathering in the following files from the Guidelines and Specs:

-     page beginning (<pb>)
-     column beginning (<cb>)
-     att.breaking
-     parameter (<param>)
-     model (<model>)
-     att.edition
-     att.milestoneUnit
-     WD Non Standard Characters
-     SG Gentle Introduction
-     PH Primary Sources
-     NH Non Hierarchical
-     HD Header
-     FT Tables Formulae Graphics
-     DS Default Text Structure
-     DI Print Dictionaries
-     CO Core Elements
-     CH Language Character Sets

There were no mentions of 'break' in the prose of but I deleted an superfluous space in the prose

The requested changes to <lb> are addressed in PR #2633.