Fix guide section about assets precompiling task in production [ci skip]

You need to specify `RAILS_ENV=production`

Related to https://github.com/rails/sass-rails/issues/152.

/cc @guilleiguaran
Merge pull request #10378 from frodsan/fix_precompiling_assets_section

Fix assets precompiling in production guide
add missing :nodoc: marks to ActiveRecord::Delegation [ci skip]
Merge pull request #10379 from frodsan/fix_nodoc_ar_delegation

add missing :nodoc: marks to ActiveRecord::Delegation [ci skip]