Arch Linux

you need to add make getpo to the install

Which locale? docs are definitely installed and help menu works fine here. Also, what is "getpo"?

https://forum.kde.org/viewtopic.php?f=210&t=151129

en_US.UTF-8 locale.

It says the usual "missing" dialogue

do you have khlelpcenter installed?

yes as well as kdoctools docbooks-xsl and docbooks-xml

what is "the usual missing dialogue"? Please post the exact, literal, message you get as well as all relevant console output.

khelp center:

Documentation not Found
Jack Ostroff <ostroffjh@users.sourceforge.net>
Revision Frameworks 5.0 (2014-04-02)
The requested documentation was not found on your computer.
The documentation may not exist, or it may not have been installed with the application.
How to solve this issue
If the application is KDE software, start by searching the KDE Documentation site for the requested documentation. If you find the documentation on that site, your distribution might ship a separate package for documentation (e.g. called plasma-doc for documentation related to Plasma). Please use the package manager of your distribution to find and install the missing documentation.
If you use a source based distribution, such as Gentoo, be sure that there are not any configuration settings (USE flags in Gentoo) that might have disabled the installation of the documentation.
If you have done that, but still get this page displayed instead of the application handbook, you probably found a bug in the help system. In this case, please report this on the KDE Bug Tracker under the KIO product.
If you do not find any documentation on the KDE Documentation site, the application may not have offline documentation. Please report this on the KDE Bug Tracker under the product for the application.
In case the application does not have offline documentation, you should use the online resources UserBase Documentation and KDE Community Forums to get help.
For non-KDE applications, please contact the application author to determine whether there should be offline documentation available.

console:
##WARNING: libpng warning: iCCP: known incorrect sRGB profile
##WARNING: libpng warning: iCCP: known incorrect sRGB profile
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "khelpcenter"
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "khelpcenter"
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "khelpcenter"
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "khelpcenter"
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "glossentry"

OK, it looks like the en_US docs are actually missing. This is due to a typo in CMakeLists.txt: if(KF5DOCTOOLS_FOUND) instead of if(KF5DocTools_FOUND)

Fixed upstream in https://cgit.kde.org/skrooge.git/commit/CMakeLists.txt?id=4d1f422b65fee6252787441b344571d5de3dd425