langref: improve packed struct memory layout description

doc/langref.html.in

@@ -2180,16 +2180,20 @@ or
       Unlike normal structs, {#syntax#}packed{#endsyntax#} structs have guaranteed in-memory layout:
       </p>
       <ul>
-        <li>Fields remain in the order declared, least to most significant.</li>
+        <li>Fields are arranged in the order declared, from the least to the most significant bits of a backing integer.</li>
         <li>There is no padding between fields.</li>
-        <li>Zig supports arbitrary width {#link|Integers#} and although normally, integers with fewer
-        than 8 bits will still use 1 byte of memory, in packed structs, they use
-        exactly their bit width.
+        <li>The backing integer is subject to the same rules as any integer, including {#link|alignment|Alignment#}, 
+        having a maximum bit count of 65535, and the host endianness.
+        On a big endian system, the first declared field will have the highest memory address.
         </li>
+        <li>The backing integer has the same bit count as the fields' total bit width.</li> 
+        <li>Field access and assignment can be understood as shorthand for bitshifts on the backing integer.</li>
+        <li>Integers will use exactly as many bits as they have. For example, a {#syntax#}u5{#endsyntax#} will use 5 bits of the backing integer.
         <li>{#syntax#}bool{#endsyntax#} fields use exactly 1 bit.</li>
         <li>An {#link|enum#} field uses exactly the bit width of its integer tag type.</li>
         <li>A {#link|packed union#} field uses exactly the bit width of the union field with
         the largest bit width.</li>
+        <li>Packed structs, when they themselves are within a packed struct, will only use as many bits as their backing integer.</li>
         <li>Packed structs support equality operators.</li>
       </ul>
       <p>
@@ -2200,8 +2204,7 @@ or
       {#code|test_packed_structs.zig#}
 
       <p>
-      The backing integer is inferred from the fields' total bit width.
-      Optionally, it can be explicitly provided and enforced at compile time:
+      The backing integer can be inferred or explicitly provided. When inferred it will be unsigned, and when explicitly provided, its bit count will be enforced at compile time:
       </p>
       {#code|test_missized_packed_struct.zig#}
 
@@ -2248,9 +2251,6 @@ or
       {#code|test_packed_struct_equality.zig#}
 
       <p>
-      Using packed structs with {#link|volatile#} is problematic, and may be a compile error in the future.
-      For details on this subscribe to
-      <a href="https://github.com/ziglang/zig/issues/1761">this issue</a>.
       TODO update these docs with a recommendation on how to use packed structs with MMIO
       (the use case for volatile packed structs) once this issue is resolved.
       Don't worry, there will be a good solution for this use case in zig.