The COMPILE_DEFINITIONS escaping disclaimer was represented in builtin
documentation using a preprocessor macro. Factor the duplicate content
out into a separate .txt file and include it in each document with the
reStructuredText include directive.
Portions of these documents contain duplicate content generated from
preprocessor macros in the original builtin documentation source.
Factor the common parts out into .txt files using reStructuredText
include directives and substitutions to avoid duplication.