summary: align left, not align middle

aligning along the left is, I think, what most projects want to do.
Aligning along the middle looks subjectively ugly, and objectively
prevents me from further indenting an element, e.g.

  Build information:
    prefix                : /usr
    sysconfdir            : /etc
        conf file         : /etc/myprogram.conf

2 files changed, 11 insertions, 7 deletions

diff --git a/docs/markdown/Reference-manual.md b/docs/markdown/Reference-manual.md
index 525c3da..802002b 100644
--- a/docs/markdown/Reference-manual.md
+++ b/docs/markdown/Reference-manual.md
@@ -1344,16 +1344,16 @@ Output:
 My Project 1.0
 
   Directories
-             prefix: /opt/gnome
-             bindir: bin
-             libdir: lib/x86_64-linux-gnu
-            datadir: share
+    prefix         : /opt/gnome
+    bindir         : bin
+    libdir         : lib/x86_64-linux-gnu
+    datadir        : share
 
   Configuration
-       Some boolean: False
+    Some boolean   : False
     Another boolean: True
-        Some string: Hello World
-             A list: string
+    Some string    : Hello World
+    A list         : string
                      1
                      True
 ```
diff --git a/docs/markdown/snippets/summary_alignment.md b/docs/markdown/snippets/summary_alignment.md
new file mode 100644
index 0000000..78a7541
--- /dev/null
+++ b/docs/markdown/snippets/summary_alignment.md
@@ -0,0 +1,4 @@
+## summary() now uses left alignment for all values
+
+Previously it aligned toward the center, but this was deemed harder to read
+than all left aligned.