[doc] Update wording regarding transmute macros (#1662)

Author: joshlf

README.md

@@ -42,7 +42,7 @@ Zerocopy provides four derivable traits for zero-cost conversions:
 - `IntoBytes` indicates that a type may safely be converted *to* a byte
   sequence
 
-This traits support sized types, slices, and [slice DSTs][slice-dsts].
+These traits support sized types, slices, and [slice DSTs][slice-dsts].
 
 [slice-dsts]: KnownLayout#dynamically-sized-types
 
@@ -61,7 +61,7 @@ You should generally derive these marker traits whenever possible.
 
 ###### Conversion Macros
 
-Zerocopy provides four macros for safe, zero-cost casting between types:
+Zerocopy provides six macros for safe casting between types:
 
 - (`try_`[try_transmute])`transmute` (conditionally) converts a value of
   one type to a value of another type of the same size
@@ -72,9 +72,12 @@ Zerocopy provides four macros for safe, zero-cost casting between types:
   mutable or immutable reference of one type to an immutable reference of
   another type of the same size
 
-These macros perform *compile-time* alignment and size checks, but cannot be
-used in generic contexts. For generic conversions, use the methods defined
-by the [conversion traits](#conversion-traits).
+These macros perform *compile-time* size and alignment checks, meaning that
+unconditional casts have zero cost at runtime. Conditional casts do not need
+to validate size or alignment runtime, but do need to validate contents.
+
+These macros cannot be used in generic contexts. For generic conversions,
+use the methods defined by the [conversion traits](#conversion-traits).
 
 ###### Byteorder-Aware Numerics
 

src/lib.rs

@@ -42,7 +42,7 @@
 //! - [`IntoBytes`] indicates that a type may safely be converted *to* a byte
 //!   sequence
 //!
-//! This traits support sized types, slices, and [slice DSTs][slice-dsts].
+//! These traits support sized types, slices, and [slice DSTs][slice-dsts].
 //!
 //! [slice-dsts]: KnownLayout#dynamically-sized-types
 //!
@@ -61,7 +61,7 @@
 //!
 //! ##### Conversion Macros
 //!
-//! Zerocopy provides four macros for safe, zero-cost casting between types:
+//! Zerocopy provides six macros for safe casting between types:
 //!
 //! - ([`try_`][try_transmute])[`transmute`] (conditionally) converts a value of
 //!   one type to a value of another type of the same size
@@ -72,9 +72,12 @@
 //!   mutable or immutable reference of one type to an immutable reference of
 //!   another type of the same size
 //!
-//! These macros perform *compile-time* alignment and size checks, but cannot be
-//! used in generic contexts. For generic conversions, use the methods defined
-//! by the [conversion traits](#conversion-traits).
+//! These macros perform *compile-time* size and alignment checks, meaning that
+//! unconditional casts have zero cost at runtime. Conditional casts do not need
+//! to validate size or alignment runtime, but do need to validate contents.
+//!
+//! These macros cannot be used in generic contexts. For generic conversions,
+//! use the methods defined by the [conversion traits](#conversion-traits).
 //!
 //! ##### Byteorder-Aware Numerics
 //!