Add alt text for svg_stroke images (#22852)

* Add alt text for svg_stroke images

Relates to #21616 
Updated line 173/174/175 https://docs.google.com/spreadsheets/d/1bUHax8ztRFt_mFaZaZAf66zdAgOVFlKMx9wMhJttQQA/edit#gid=790929018

* Apply suggestions from code review

Co-authored-by: Estelle Weyl <estelle@openwebdocs.org>

files/en-us/web/svg/tutorial/fills_and_strokes/index.md

@@ -32,7 +32,7 @@ In addition, you can specify the opacity of either the `fill` or `stroke` separa
 
 In addition to its color properties, there are a few other attributes available to control the way a stroke is drawn on a line.
 
-![](svg_stroke_linecap_example.png)
+![The stroke-linecap attribute changes the look of these stroke's ends: square adds a square cap, round provides a rounded cap, and butt removes capping](svg_stroke_linecap_example.png)
 
 ```xml
 <?xml version="1.0" standalone="no"?>
@@ -55,7 +55,7 @@ There are three possible values for `stroke-linecap`:
 
 Use `stroke-linejoin` to control how the joint between two line segments is drawn.
 
-![](svg_stroke_linejoin_example.png)
+![The stroke-linejoin attribute changes the look at the point where two lines join, with miter created an angled join, round rounding the corner, and bevel creating a beveled edge, flattening the corner .](svg_stroke_linejoin_example.png)
 
 ```xml
 <?xml version="1.0" standalone="no"?>
@@ -75,7 +75,7 @@ Each of these polylines has two segments. The joint where the two meet is contro
 
 Finally, you can also use dashed line types on a stroke by specifying the `stroke-dasharray` attribute.
 
-![](svg_stroke_dasharray_example.png)
+![Two custom dashed lines, one with evenly spaced dashes and the other using a long-dash short dash using a stroke-dasharray attribute value.](svg_stroke_dasharray_example.png)
 
 ```xml
 <?xml version="1.0" standalone="no"?>