Fixes textdomains from 'buddypress' to 'bp-docs'

Props patloq on wordpress.org

See http://wordpress.org/support/topic/minor-translation-issue#post-4608405
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Allow empty content field when creating/editing

Fixes #358. Fixes #356

Props @dcavins
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Use bp_docs_get_docs_slug() instead of constant throughout

In order to have a reliably customizable slug, it's inefficient to
reference the constant directly. Instead, use bp_docs_get_docs_slug().

See #346
Don't define BP_DOCS_SLUG up front. See #346
Merge upstream changes