The help/encyclopedia text entries are found in our game's Resources/Entities/language.str file. This file can be opened with any plain-text editor, but we recommend using StringWrangler, the program we developed to edit .str files. It will show a list of all strings in the file, filterable by a text entry just over the list, where you can click on any of them and have the contents of the string show up for copy/pasting.
All faction and brigade files have an article subtag that has the base of their string; the string you're looking for will be the contents of that tag followed by ::TOOLTIP, e.g. factions/rome.xnt has <article>Factions::Rome</article>, so the string to look up for them would be Factions::Rome::TOOLTIP.