[petsc-dev] Users manual update

[Jed Brown]

huabel via petsc-dev <petsc-dev at mcs.anl.gov> writes:

> Thanks, I copied the ‘developer’ fold out and comment (#extensions.append('sphinxcontrib.bibtex’) this line, with few copy it works well, (no cite link), for me it enough. 

We recommend using `make sphinx-docs-html` from the top-level.  It will create/activate a virtualenv based on requirements.txt so everything will work.  It's fast for incremental updates.