ExternalProject: Convert docs to a bracket comment

Use a bracket comment to hold the documentation instead of a block of
line comments.  This will make further updates easier.  Also fix the
logic that parses options out of the documentation to be aware of the
new layout.
ExternalProject: Use explicit markup and definition lists in docs

Convert ExternalProject module documentation to use explicit markup
blocks for each command.  Enumerate command options with definition
lists instead of a literal block.  This will allow more detail to be
added later.  Also fix the logic that parses options out of the
documentation to be aware of the new layout.
Merge topic 'ExternalProject-format-docs' into next

d9c2c17b ExternalProject: Use explicit markup and definition lists in docs
98936ae3 ExternalProject: Convert docs to a bracket comment