spec: clarify section on string types

Strings happen to be represented similarly to
byte slices internally, but they don't quite
behave like them: While strings can be indexed,
sliced, and have their len() taken like byte
slices, string elements are not addressable,
make() and cap() is not supported, range loops
operate differently, and they are immutable (and
thus behave like values rather then references).

Fixes #4018.

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/6503116

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of September 12, 2012",
+	"Subtitle": "Version of September 13, 2012",
 	"Path": "/ref/spec"
 }-->
 
@@ -781,19 +781,21 @@ particular architecture.
 
 <p>
 A <i>string type</i> represents the set of string values.
-Strings behave like slices of bytes but are immutable: once created,
+A string value is a (possibly empty) sequence of bytes.
+Strings are immutable: once created,
 it is impossible to change the contents of a string.
 The predeclared string type is <code>string</code>.
+</p>
 
 <p>
-The elements of strings have type <code>byte</code> and may be
-accessed using the usual <a href="#Indexes">indexing operations</a>.  It is
-illegal to take the address of such an element; if
-<code>s[i]</code> is the <i>i</i>th byte of a
-string, <code>&amp;s[i]</code> is invalid.  The length of string
-<code>s</code> can be discovered using the built-in function
-<code>len</code>. The length is a compile-time constant if <code>s</code>
-is a string literal.
+The length of a string <code>s</code> (its size in bytes) can be discovered using
+the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
+The length is a compile-time constant if the string is a constant.
+A string's bytes can be accessed by integer indices 0 through
+<code>len(s)-1</code> (§<a href="#Indexes">Indexes</a>).
+It is illegal to take the address of such an element; if
+<code>s[i]</code> is the <code>i</code>'th byte of a
+string, <code>&amp;s[i]</code> is invalid.
 </p>
 
 
@@ -816,7 +818,7 @@ ElementType = Type .
 The length is part of the array's type and must be a
 <a href="#Constant_expressions">constant expression</a> that evaluates to a non-negative
 integer value.  The length of array <code>a</code> can be discovered
-using the built-in function <a href="#Length_and_capacity"><code>len(a)</code></a>.
+using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
 The elements can be indexed by integer
 indices 0 through <code>len(a)-1</code> (§<a href="#Indexes">Indexes</a>).
 Array types are always one-dimensional but may be composed to form
@@ -847,7 +849,7 @@ SliceType = "[" "]" ElementType .
 <p>
 Like arrays, slices are indexable and have a length.  The length of a
 slice <code>s</code> can be discovered by the built-in function
-<a href="#Length_and_capacity"><code>len(s)</code></a>; unlike with arrays it may change during
+<a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
 execution.  The elements can be addressed by integer indices 0
 through <code>len(s)-1</code> (§<a href="#Indexes">Indexes</a>).  The slice index of a
 given element may be less than the index of the same element in the
@@ -1249,7 +1251,7 @@ map[string]interface{}
 <p>
 The number of map elements is called its length.
 For a map <code>m</code>, it can be discovered using the
-built-in function <a href="#Length_and_capacity"><code>len(m)</code></a>
+built-in function <a href="#Length_and_capacity"><code>len</code></a>
 and may change during execution. Elements may be added during execution
 using <a href="#Assignments">assignments</a> and retrieved with
 <a href="#Indexes">index</a> expressions; they may be removed with the