Update TARGETPATH and PATHPATTERN recommendations

In addition to recommending the omission of leading directory
separators, this commit adds a recommendation for forward slashes
as preferred directory separators in TARGETPATH and PATHPATTERN.

The rationale is that a TARGETPATH needs to be concatenated with
a mirror's base URL, and as such it might yield unexpected
behavior with non standardized paths. Same goes for Unix
shell-style pattern matching of target paths using PATHPATTERN.

As suggested by @erickt this commit adds a reference to the
"path-relative-URL string" definition from the WHATWG URL spec as
the preferred TARGETPATH style.

This commit also switches to using the term "directory separator"
instead of the prior used "path separator" to disambiguate from
characters that separate paths (e.g. colon on Unix).

Author: lukpueh

tuf-spec.md

@@ -806,9 +806,13 @@ repo](https://github.com/theupdateframework/specification/issues).
        }
 
    Each key of the TARGETS object is a TARGETPATH.  A TARGETPATH is a path to
-   a file that is relative to a mirror's base URL of targets. It should not
-   have a leading path separator to avoid surprising behavior when constructing
-   paths on disk.
+   a file that is relative to a mirror's base URL of targets. To avoid
+   surprising behavior when resolving paths, it is RECOMMENDED that a
+   TARGETPATH uses the forward slash (/) as directory separator and does not
+   start with a directory separator. The recommendation for TARGETPATH aligns
+   with the ["path-relative-URL string"
+   definition](https://url.spec.whatwg.org/#path-relative-url-string) in the
+   WHATWG URL specification.
 
    It is allowed to have a TARGETS object with no TARGETPATH elements.  This
    can be used to indicate that no target files are available.
@@ -881,8 +885,11 @@ repo](https://github.com/theupdateframework/specification/issues).
    match file paths "targets/foo.tgz" and "targets/bar.tgz", but not
    "targets/foo.txt".  Likewise, path pattern "foo-version-?.tgz" matches
    "foo-version-2.tgz" and "foo-version-a.tgz", but not "foo-version-alpha.tgz".
-   It should not have a leading path separator to avoid surprising behavior when
-   constructing paths on disk.
+   To avoid surprising behavior when matching targets with PATHPATTERN, it is
+   RECOMMENDED that PATHPATTERN uses the forward slash (/) as directory
+   separator and does not start with a directory separator, akin to
+   TARGETSPATH.
+
 
    Prioritized delegations allow clients to resolve conflicts between delegated
    roles that share responsibility for overlapping target paths.  To resolve