Update CTCDecoder static build deprecation message (#3089)

Summary:
1. Fix spacing.
2. Move it to after successful import
3. Add link to the announcement issue

Pull Request resolved: https://github.com/pytorch/audio/pull/3089

Reviewed By: nateanl, xiaohui-zhang

Differential Revision: D43514075

Pulled By: mthrok

fbshipit-source-id: 3b2a24c65c63dab8c12c9c6aa1942a8354b2c0f1

torchaudio/models/decoder/_ctc_decoder.py

@@ -39,10 +39,6 @@ if torchaudio._internal.module_utils.is_module_available("flashlight"):
     except Exception:
         _KenLM = None
 else:
-    warnings.warn(
-        "The built-in flashlight integration is deprecated, and will be removed in future release."
-        "Please install flashlight-text. https://pypi.org/project/flashlight-text/"
-    )
     torchaudio._extension._load_lib("libflashlight-text")
     from torchaudio.lib.flashlight_lib_text_decoder import (
         CriterionType as _CriterionType,
@@ -63,6 +59,12 @@ else:
         load_words as _load_words,
     )
 
+    warnings.warn(
+        "The built-in flashlight integration is deprecated, and will be removed in future release. "
+        "Please install flashlight-text. https://pypi.org/project/flashlight-text/ "
+        "For the detail of CTC decoder migration, please see https://github.com/pytorch/audio/issues/3088."
+    )
+
 
 __all__ = [
     "CTCHypothesis",