user-search: Remove approval requirement from basic search

Basic search (i.e. not using the explicit predicate) can never work
as expected for approval requirements because when a label is applied
to a change a comment containing the label name is added as a message
on the change. This means that any change with that label will match
with the comment predicate, regardless of the label score.

For example if we have two changes with the following labels:

  Change 1: Code-Review+2
  Change 2: Code-Review+1

Then both changes have messages like:

  Patch Set 1: Code-Review+2
  Patch Set 1: Code-Review+1

And when we issue the query:

  Code-Review+2

the query builder resolves it into both a label predicate (which will
only match change 1) and a message predicate (which will match both
changes).

Undo the formatting change that was done in Ia1a138d3e, since it is no
longer relevant.

Instead, remove the approval requirement section from the basic search
documentation.

Add a clarification that if the user wants predictable search results
they should use explicit predicates rather than the basic search.

This reverts commit 025b2633aefaa04eba8134dee90da559efd94bcd.

Bug: Issue 8589
Change-Id: I58a27530c3f2b530d078e030b2eebd3e47c82f0f