First, beware of the term Master Document. You want nothing to do with a Word Master Document. Master Documents "Feature"

Look into AutoText and AutoText fields.
Automated Boilerplate Using Microsoft Word (AutoText, AutoCorrect, Building Blocks, and more)

For individual documents that need to repeat custom text in multiple places (i.e. names, titles, amounts, dates) look at Repeating Data Using Document Properties Content Controls and Other Mapped Content Controls.