Basically, this scans a portion of the supplied string, checking to see
if it has a tag-like structure. It is possible to "confuse" this, which
may subsequently result in parse errors, but in the vast majority of
cases, this method serves as a valid inicator of whether or not the
content looks like XML.
Things that are intentional excluded:
- plain text with no markup.
- strings that look like filesystem paths.
Subclasses SHOULD NOT OVERRIDE THIS. Altering it may be altering
core assumptions about how things work. Instead, classes should
override the constructor and pass in only one of the parsed types
that this class expects.
