In Windows, each jpeg has "Properties", and in the "Properties" dialogue box, there are General, Security & Details tabs. On the Details tab is an area for Comments. It is these Comments that I would like to be included when I sync and/or import into Lightroom 5.5.
This is a non-industry-standard metadata field introduced by Windows XP, and most other software (including LR) ignores it. If you'd like to import that field into LR, you could use Exiftool (free) to copy (or append) that field into the field that LR will read as its caption (XMP:Description). Exiftool has the ability to process many photos all at once and make backups of the originals.
While viewing the Metadata for a picture in Lightroom (Library module), there is a Comments area, but my screen shows the message "Selected Service Does Not Support Comments".
LR has the ability to publish collections of pics to services like Flickr or Facebook and sync those collections with the services. If, for example, a Flickr user adds a comment at Flickr.com to a pic that's being synced with a LR published collection, LR will download the comment and show it in the Comments area. This feature has very limited usefulness, since you can't copy text from the Comments field (!) and you can't search the field, or even search for photos that have comments.
Thank you, John Ellis. I appreciate you taking the time to provide a well written and accurate answer.