Minor cleanup/docs

(cherry picked from commit 44d6c4fe440b6a44de3fa90a99b01652660188f3)

source-api/src/commonMain/kotlin/eu/kanade/tachiyomi/source/online/HttpSource.kt

@@ -135,6 +135,7 @@ abstract class HttpSource : CatalogueSource {
      *
      * @param page the page number to retrieve.
      */
+    @Deprecated("Use the non-RxJava API instead", replaceWith = ReplaceWith("getPopularManga"))
     override fun fetchPopularManga(page: Int): Observable<MangasPage> {
         return client.newCall(popularMangaRequest(page))
             .asObservableSuccess()
@@ -165,6 +166,7 @@ abstract class HttpSource : CatalogueSource {
      * @param query the search query.
      * @param filters the list of filters to apply.
      */
+    @Deprecated("Use the non-RxJava API instead", replaceWith = ReplaceWith("getSearchManga"))
     override fun fetchSearchManga(
         page: Int,
         query: String,
@@ -209,6 +211,7 @@ abstract class HttpSource : CatalogueSource {
      *
      * @param page the page number to retrieve.
      */
+    @Deprecated("Use the non-RxJava API instead", replaceWith = ReplaceWith("getLatestUpdates"))
     override fun fetchLatestUpdates(page: Int): Observable<MangasPage> {
         return client.newCall(latestUpdatesRequest(page))
             .asObservableSuccess()