style-guide: Move text about block vs visual indent to indentation section

`principles.md` includes some high-level guiding principles for
formatting, but also includes a few specific formatting provisions.
While those provisions apply in many places, the same holds true for
other high-level guidance, such as the indentation section. Move the
text about using block indent rather than visual indent to the
indentation section, so that `principles.md` can focus on guiding
principles while the top level of the style guide gives concrete
formatting recommendations.

Author: joshtriplett

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

@@ -30,6 +30,24 @@ typically by using a formatting tool's default settings.
 * The maximum width for a line is 100 characters.
 * A tool should be configurable for all three of these variables.
 
+#### Block indent
+
+Prefer block indent over visual indent:
+
+```rust
+// Block indent
+a_function_call(
+    foo,
+    bar,
+);
+
+// Visual indent
+a_function_call(foo,
+                bar);
+```
+
+This makes for smaller diffs (e.g., if `a_function_call` is renamed in the above
+example) and less rightward drift.
 
 ### Blank lines
 

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

@@ -31,23 +31,6 @@ following principles (in rough priority order):
 
 ## Overarching guidelines
 
-Prefer block indent over visual indent. E.g.,
-
-```rust
-// Block indent
-a_function_call(
-    foo,
-    bar,
-);
-
-// Visual indent
-a_function_call(foo,
-                bar);
-```
-
-This makes for smaller diffs (e.g., if `a_function_call` is renamed in the above
-example) and less rightward drift.
-
 Lists should have a trailing comma when followed by a newline, see the block
 indent example above. This choice makes moving code (e.g., by copy and paste)
 easier and makes smaller diffs.