Work around a hang

[zickgraf]

as suggested by @KristofferC in JuliaDocs/Documenter.jl#2121 (comment).

I have verified that this fixes JuliaDocs/Documenter.jl#2121. The yield could also be moved to line 109 (or anywhere in between), but I thought right in front of the place where the hang actually occurs (i.e. the call to f()) would be most fitting.

[mortenpi]

LGTM, it definitely seems to take care of the hang. Could you also please:

1. Add a note to CHANGELOG.md about this (under v0.2.3; with an issue reference to here, and to the Documenter issue).
2. Bump the version number in Project.toml

[zickgraf]

LGTM, it definitely seems to take care of the hang. Could you also please:

1. Add a note to `CHANGELOG.md` about this (under `v0.2.3`; with an issue reference to here, and to the Documenter issue).

2. Bump the version number in `Project.toml`

Done. I have also added a test while at it.

[mortenpi]

Awesome, thank you!