doc, api: add type Srcset string change to go1.10 notes and API

Change-Id: I13afaa894292bd566510b40a5e4bbbe4d72d4d08
Reviewed-on: https://go-review.googlesource.com/87395Reviewed-by: Ian Lance Taylor <iant@golang.org>

api/go1.10.txt

@@ -573,6 +573,7 @@ pkg encoding/xml, type TokenReader interface, Token() (Token, error)
 pkg flag, method (*FlagSet) ErrorHandling() ErrorHandling
 pkg flag, method (*FlagSet) Name() string
 pkg flag, method (*FlagSet) Output() io.Writer
+pkg html/template, type Srcset string
 pkg math, func Erfcinv(float64) float64
 pkg math, func Erfinv(float64) float64
 pkg math, func Round(float64) float64

doc/go1.10.html

@@ -1050,6 +1050,12 @@ The new actions <code>{{"{{break}}"}}</code> and <code>{{"{{continue}}"}}</code>
 break out of the innermost <code>{{"{{range"}}</code>&nbsp;...<code>}}</code> loop,
 like the corresponding Go statements.
 </p>
+<p>
+The new <a href="/pkg/html/template#Srcset"><code>Srcset</code></a> content
+type allows for proper handling of values within the
+<a href="https://w3c.github.io/html/semantics-embedded-content.html#element-attrdef-img-srcset"><code>srcset</code></a>
+attribute of <code>img</code> tags.
+</p>
 </dl>
 
 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>