Don't reset view selection on :if and :else

This makes a bit more complex scripting possible.  Otherwise %f
becomes %c inside if statement.
Add missing docs on cmdline commands to man page

Just one small paragraph, which was present in vim-doc
Document selection resetting by cmdline commands
Move udf completion test to commands tests
Change need_clean_selection -> keep_view_selection

This is easier to understand and search for.
Merge branch 'if-else-and-selection'

Don't reset view selection on :if and :else commands.

This makes a bit more complex scripting possible.  Otherwise %f becomes
%c inside if statement.