usrguide.pdf: Mention space-trimmed aspect of \NewDocumentEnvironment et al.

[dr-scsi]

ltnews40 contains:

Fix existence check of document environments

\NewDocumentEnvironment and friends define (or redefine) a document environment
using the space-trimmed <envname> [...]. For example, in
\NewDocumentEnvironment{myenv}{}{begin}{end}
\NewDocumentEnvironment{ myenv }{}{begin}{end}
[...]

usrguide.pdf doesn't mention this aspect of the macros; I think it should be pointed out that there since this behavior is really different from what \newenvironment does.