Clarify append deprecation notice.

[bdice]

This clarifies that the deprecated append method is replaced by a top-level function cudf.concat. This has caused some user confusion, because it's not just a name change (DataFrame.concat doesn't exist).

[shwina]

Thanks @bdice! Could you please change this to target 22.06 instead?

[codecov]

Codecov Report

Merging #10930 (3be1983) into branch-22.06 (54789ee) will increase coverage by 0.02%.
The diff coverage is 93.75%.

@@               Coverage Diff                @@
##           branch-22.06   #10930      +/-   ##
================================================
+ Coverage         86.30%   86.32%   +0.02%     
================================================
  Files               144      144              
  Lines             22665    22668       +3     
================================================
+ Hits              19560    19569       +9     
+ Misses             3105     3099       -6     

Impacted Files	Coverage Δ
python/cudf/cudf/core/indexed_frame.py	91.70% <ø> (ø)
python/cudf/cudf/utils/ioutils.py	79.47% <87.50%> (-0.13%)	⬇️
python/cudf/cudf/io/avro.py	78.57% <100.00%> (ø)
python/cudf/cudf/io/csv.py	91.80% <100.00%> (ø)
python/cudf/cudf/io/json.py	97.56% <100.00%> (ø)
python/cudf/cudf/io/orc.py	92.77% <100.00%> (ø)
python/cudf/cudf/io/parquet.py	90.83% <100.00%> (ø)
python/cudf/cudf/io/text.py	100.00% <100.00%> (ø)
python/cudf/cudf/core/dataframe.py	93.78% <0.00%> (+0.04%)	⬆️
python/cudf/cudf/core/column/string.py	88.78% <0.00%> (+0.12%)	⬆️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5067cc7...3be1983. Read the comment docs.

[vyasr]

@gpucibot merge