start writing vram_classes docs

docs/file_format/segments.md

@@ -128,7 +128,7 @@ The name of another segment.
 
 If used then force putting the segment at address of the given vram class.
 
-See the [`vram_class`](vram_class.md) docs for more info.
+See the [`vram_classes`](vram_classes.md) docs for more info.
 
 It can't be used in combination with [`fixed_vram`](#fixed_vram),
 [`fixed_symbol`](#fixed_symbol) or [`follows_segment`](#follows_segment).

docs/file_format/vram_classes.md

@@ -0,0 +1,16 @@
+# Vram classes
+
+Vram classes aim to help reduce duplicated data across the segments and more
+clearly organize the memory layout.
+
+A vram class specifies a place in memory, which may be described in a somewhat
+complex way, so one or more segments can point to this class instead of try to
+describe the memory layout on the segment itself.
+
+## `name`
+
+## `fixed_vram`
+
+## `fixed_symbol`
+
+## `follow_classes`