[type-layout] Document minimum size and alignment

src/type-layout.md

@@ -32,6 +32,29 @@ as [dynamically sized types]. Since all values of a `Sized` type share the same
 size and alignment, we refer to those shared values as the size of the type and
 the alignment of the type respectively.
 
+### Minimum Size and Alignment
+
+Every type, including [dynamically sized types], has a "minimum size" and "minimum
+alignment." Every value of a type must have a size at least as large as the type's
+minimum size and an alignment at least as large as the type's minimum alignment.
+Some notable cases of minimum size and alignment are:
+- For [`Sized`] types, the type's minimum size and minimum alignment
+  are always equal to the type's size and alignment, respectively
+- For a [slice type](#slice-layout), `[T]`, the minimum size is 0 bytes (corresponding
+  to a slice with 0 elements), and the minimum alignment is the alignment of `T`
+- For a [trait object](#trait-object-layout), the minimum size is 0 and the minimum
+  alignment is 1
+- For a struct type with a dynamically-sized field, the minimum size is taken to be
+  the size of the struct when the dynamically-sized field has *its* minimum size and
+  its minimum alignment. The struct's minimum alignment is taken to be the alignment
+  of the struct when the dynamically-sized field has *its* minimum alignment. When
+  relying on this property of dynamically-sized structs, be careful not to assume too
+  much! For example, this property provides *no* guarantees about
+  [`repr(Rust)`](#the-rust-representation) types, as such a type can have arbitrarily
+  large size and alignment regardless of the sizes and alignments of its fields
+
+Every type's minimum size is less than or equal to `isize::MAX`.
+
 ## Primitive Data Layout
 
 The size of most primitives is given in this table.