This adds some clarifications on the time unit strings the function
takes as arguments, noting the differences between these and the time
span literals, as well as the abbreviations' source.
This modifies the ESQL test infrastructure to generate more of the
documentation for functions. It generates the *Description* section, the
*Examples* section, and the *Parameters* section as separate files so we
can use them as needed. It also generates a `layout` file that's just
a guess as to how to render the whole thing. In some cases it'll work
and we can use that instead of hand maintaining a "top level"
description file for the function.
Most newly generated files are unused. We have to chose to pick them up
by replacing the sections we were manually maintaining with an include
of the generated section. Or by replacing the entire hand maintained
file with the generated top level file.
Relates to #104247
