hst: improve doc comments

These now read a lot better both in source and on pkgsite.

Signed-off-by: Ophestra <cat@gensokyo.uk>

hst/fsoverlay.go

@@ -19,9 +19,11 @@ type FSOverlay struct {
 
 	// Any filesystem, does not need to be on a writable filesystem, must not be nil.
 	Lower []*check.Absolute `json:"lower"`
-	// The upperdir is normally on a writable filesystem, leave as nil to mount Lower readonly.
+	// The upperdir is normally on a writable filesystem, leave as nil to mount
+	// Lower readonly.
 	Upper *check.Absolute `json:"upper,omitempty"`
-	// The workdir needs to be an empty directory on the same filesystem as Upper, must not be nil if Upper is populated.
+	// The workdir needs to be an empty directory on the same filesystem as
+	// Upper, must not be nil if Upper is populated.
 	Work *check.Absolute `json:"work,omitempty"`
 }