Fix SDSS-I/II spSpec units (astropy#1066)

* Add test for checking SDSS wavelength

* Enforce Angstroms in SDSS

* Add links to SDSS-I/II spectra formats docs

This should mean that future editors of those loaders (e.g. to add new
features or fix bugs) know what they can assume about the files.

specutils/io/default_loaders/sdss.py

@@ -30,7 +30,10 @@ def _sdss_wcs_to_log_wcs(old_wcs):
     claims to be linear, but is logarithmic in base-10.
     The wavelength is given by:
     λ = 10^(w0 + w1 * i)
-    with i being the pixel index starting from 0.
+    with i being the pixel index starting from 0. This formula is documented at
+    https://classic.sdss.org/dr7/products/spectra/read_spSpec.php and appears to
+    be the same across all SDSS-I and SDSS-II data releases (replace dr7 with
+    dr<x> in the above URL).
 
     The FITS standard uses a natural log with a sightly different formulation,
     see WCS Paper 3 (which discusses spectral WCS).
@@ -41,7 +44,7 @@ def _sdss_wcs_to_log_wcs(old_wcs):
     w1 = old_wcs.wcs.cd[0,0]
     crval = 10 ** w0
     cdelt = crval * w1 * np.log(10)
-    cunit = old_wcs.wcs.cunit[0] or Unit('Angstrom')
+    cunit = Unit('Angstrom')
     ctype = "WAVE-LOG"
 
     w = WCS(naxis=1)
@@ -147,6 +150,11 @@ def spSpec_loader(file_obj, **kwargs):
     """
     Loader for SDSS-I/II spSpec files.
 
+    The content of these files is documented at
+    https://classic.sdss.org/dr7/dm/flatFiles/spSpec.php, with instructions for
+    reading them at
+    https://classic.sdss.org/dr7/products/spectra/read_spSpec.php.
+
     Parameters
     ----------
     file_obj: str, file-like, or HDUList

specutils/tests/test_loaders.py

@@ -208,6 +208,7 @@ def test_sdss_spspec(tmp_path):
         spec = Spectrum1D.read(response, format="SDSS-I/II spSpec")
         assert isinstance(spec, Spectrum1D)
         assert spec.flux.size > 0
+        assert (spec.wavelength[0] / u.AA).value == pytest.approx(3799.268615)
 
     file_path = str(tmp_path / sp_pattern)
     with urllib.request.urlopen(url) as response: