For better or for worse, the guidance is that the man pages are not generated in packages. This makes sense to me for a resource-constrained device, but can be challenging for executables that aren't self-documenting (most supplied by busybox at least output a terse summary).
jeff@office:~$ ls --help
BusyBox v1.29.2 () multi-call binary.
Usage: ls [-1AaCxdLHRFplinshrSXvctu] [-w WIDTH] [FILE]...
List directory contents
-1 One column output
-a Include entries which start with .
-A Like -a, but exclude . and ..
-x List by lines
-d List directory entries instead of contents
-L Follow symlinks
-H Follow symlinks on command line
-R Recurse
-p Append / to dir entries
-F Append indicator (one of */=@|) to entries
-l Long listing format
-i List inode numbers
-n List numeric UIDs and GIDs instead of names
-s List allocated blocks
-lc List ctime
-lu List atime
--full-time List full date and time
-h Human readable sizes (1K 243M 2G)
--group-directories-first
-S Sort by size
-X Sort by extension
-v Sort by version
-t Sort by mtime
-tc Sort by ctime
-tu Sort by atime
-r Reverse sort order
-w N Format N columns wide
--color[={always,never,auto}] Control coloring
When you say "OpenWrt docs", do you mean just the equivalent of man pages, or the wiki?