style-guide: Document newline rules for assignment operators

The style guide gives general rules for binary operators including
assignment, and one of those rules says to put the operator on the
subsequent line; the style guide needs to explicitly state the exception
of breaking *after* assignment operators rather than before.

This is already what rustfmt does and what users do; this fixes the
style guide to match the expected default style.

Author: joshtriplett

src/doc/style-guide/src/expressions.md

@@ -294,8 +294,9 @@ Use parentheses liberally, do not necessarily elide them due to precedence.
 Tools should not automatically insert or remove parentheses. Do not use spaces
 to indicate precedence.
 
-If line-breaking, put the operator on a new line and block indent. Put each
-sub-expression on its own line. E.g.,
+If line-breaking, block-indent each subsequent line. For assignment operators,
+break after the operator; for all other operators, put the operator on the
+subsequent line. Put each sub-expression on its own line:
 
 ```rust
 foo_bar