Fix display of labels in a [horizontal] list

The introduction of the entry for `compdb-targets` in the `[horizontal]`
labeled list in doc/manual.asciidoc revealed some display issues in the
left column:

First, the web browser would insert a line break in the middle of the
label `compdb-targets`, so that it looked like this:

       compdb-
       targets

We fix this by applying the `white-space: nowrap` attribute to the left
column.

After this is fixed, we see practically no space between the end of the
longest label and the beginning of the text in the second column; we fix
this with the `padding-right` attribute.

Finally, we align all labels to the right side of the column so that
there is a consistent amount of horizontal space between the end of each
label and the beginning of the text in the second column.

doc/style.css

@@ -53,3 +53,16 @@ div.chapter {
 p {
     margin-top: 0;
 }
+
+/* The following applies to the left column of a [horizontal] labeled list: */
+table.horizontal > tbody > tr > td:nth-child(1) {
+
+    /* prevent the insertion of a line-break in the middle of a label: */
+    white-space: nowrap;
+
+    /* insert a little horizontal padding between the two columns: */
+    padding-right: 1.5em;
+
+    /* right-justify labels: */
+    text-align: end;
+}