Add initial documentation for sort members

Change-Id: I2f097d0f9eab08679dfa7660a698ea8dc1775cf4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/507461
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server/doc/design/features/sort_members.md

@@ -0,0 +1,30 @@
+# Sort members
+
+The sort members command is used to sort both top-level and member declarations.
+
+The sort order is based on four criteria:
+
+- The kind of the declaration.
+- Whether the declaration is static.
+- The visibility of the declaration.
+- The name of the declaration.
+
+The order of the kinds is influenced by the `sort_constructors_first` lint. When
+the lint is enabled, constructors will be sorted before other members. When the
+lint isn't enabled, fields will be sorted first, with constructors following.
+
+## Field declarations
+
+When a field declaration declares more than one field, for sorting purposes, the
+name of the first field is used. For example, given the field declaration
+
+```dart
+int x, y, z;
+```
+
+The declaration will be sorted using the name `x`.
+
+## Primary constructor bodies
+
+A primary constructor body is placed in its own group, and that group always
+sorts immediately before secondary constructors.