docs: Fix formatting of definition lists

Definitions in definition lists in most markdown implementations,
including the GitLab one, support laziness for the definition text
(https://spec.commonmark.org/0.31.2/#lazy-continuation-line).
As a result, each defined term would be collapsed into preceding definition.
To fix this, definitions need to be separated by blank line.

docs/macros.md

@@ -6,14 +6,19 @@ being defined at build time:
 
 `--buildtype={plain,release,minsize,custom}`
  : No special macros or options
+
 `--buildtype={debug,debugoptimized}` (`debugoptimized` is the default)
  : `-DG_ENABLE_DEBUG -g`
+
 `-Dglib_debug=disabled`
  : Omits `G_ENABLE_DEBUG` when implied by `--buildtype`/`-Ddebug`
+
 `-Dglib_debug=enabled`
  : Defines `G_ENABLE_DEBUG` regardless of `--buildtype`/`-Ddebug`
+
 `-Dglib_asserts=false`
  : `-DG_DISABLE_ASSERT`
+
 `-Dglib_checks=false`
  : `-DG_DISABLE_CHECKS`