Honestly, I didn't try to debug the situation - just went with the quick fix. Here's a printout of the version and the error. I'm using wget on a Mac command line.
$ wget --version
GNU Wget 1.11.4
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Originally written by Hrvoje Niksic <firstname.lastname@example.org>.
Currently maintained by Micah Cowan <email@example.com>.
Here's the error I get. It appears the subdomain conflicts with the certificate, which is associated with the main domain.
$ wget -r "https://docs.syncthing.net"
--2017-02-18 07:57:08-- https://docs.syncthing.net/
Resolving docs.syncthing.net... 18.104.22.168, 2a03:b0c0:3:d0::18d6:8001
Connecting to docs.syncthing.net|22.214.171.124|:443... connected.
ERROR: cannot verify docs.syncthing.net's certificate, issued by `/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3':
Unable to locally verify the issuer's authority.
ERROR: certificate common name `syncthing.net' doesn't match requested host name `docs.syncthing.net'.
To connect to docs.syncthing.net insecurely, use `--no-check-certificate'.
Unable to establish SSL connection.