docs/meson.build: Build the library doc after the backend one

The library documentation has references to the one from the backend, so
it should be built after

This commit fixes the following warnings:

  html/GClueClient.html:349: warning: no link for: "gdbus-signal-org-freedesktop-GeoClue2-Client.LocationUpdated" -> ("LocationUpdated").
  html/GClueClient.html:386: warning: no link for: "gdbus-method-org-freedesktop-GeoClue2-Client.Start" -> (<GTKDOCLINK HREF="Start"><code class="function">Start()</code>).
  html/GClueClient.html:520: warning: no link for: "gdbus-method-org-freedesktop-GeoClue2-Client.Stop" -> (<GTKDOCLINK HREF="Stop"><code class="function">Stop()</code>).
  html/GClueClient.html:651: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Client.Location" -> ("Location").
  html/GClueClient.html:744: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Client.DistanceThreshold" -> ("DistanceThreshold").
  html/GClueClient.html:805: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Client.DesktopId" -> ("DesktopId").
  html/GClueClient.html:899: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Client.RequestedAccuracyLevel" -> ("RequestedAccuracyLevel").
  html/GClueClient.html:961: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Client.Active" -> ("Active").
  html/GClueClient.html:1071: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Client.TimeThreshold" -> ("TimeThreshold").
  html/GClueLocation.html:322: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Location.Latitude" -> ("Latitude").
  html/GClueLocation.html:383: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Location.Longitude" -> ("Longitude").
  html/GClueLocation.html:444: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Location.Accuracy" -> ("Accuracy").
  html/GClueLocation.html:505: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Location.Altitude" -> ("Altitude").
  html/GClueLocation.html:566: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Location.Speed" -> ("Speed").
  html/GClueLocation.html:627: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Location.Heading" -> ("Heading").
  html/GClueLocation.html:688: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Location.Description" -> ("Description").
  html/GClueLocation.html:781: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Location.Timestamp" -> ("Timestamp").
  html/GClueManager.html:266: warning: no link for: "gdbus-method-org-freedesktop-GeoClue2-Manager.GetClient" -> (<GTKDOCLINK HREF="GetClient"><code class="function">GetClient()</code>).
  html/GClueManager.html:413: warning: no link for: "gdbus-method-org-freedesktop-GeoClue2-Manager.AddAgent" -> (<GTKDOCLINK HREF="AddAgent"><code class="function">AddAgent()</code>).
  html/GClueManager.html:555: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Manager.InUse" -> ("InUse").
  html/GClueManager.html:617: warning: no link for: "gdbus-property-org-freedesktop-GeoClue2-Manager.AvailableAccuracyLevel" -> ("AvailableAccuracyLevel").
  html/GClueManager.html:755: warning: no link for: "gdbus-method-org-freedesktop-GeoClue2-Manager.CreateClient" -> (<GTKDOCLINK HREF="CreateClient"><code class="function">CreateClient()</code>).
  html/GClueManager.html:799: warning: no link for: "gdbus-method-org-freedesktop-GeoClue2-Manager.DeleteClient" -> (<GTKDOCLINK HREF="DeleteClient"><code class="function">DeleteClient()</code>).
3 jobs for libdoc in 3 minutes and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #423995
ubuntu-16.04

00:01:39

passed #423996
ubuntu-18.04

00:03:49

passed #423997
ubuntu-18.04-no-backend

00:01:28