DOC/CLN: `how` is completely inconsistent

[bashtage]

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Reproducible Example

import pandas as pd

Period("2000Q1").asfreq(Day(), "Finish")
Period("2000Q1").asfreq(Day(), "E")
Period("2000Q1").asfreq(Day(), "End")
Period("2000Q1").asfreq(Day(), "end")

are all the same.

### Issue Description

`Period.asfreq` and `Period.to_timestamp` both technically support the same arguments, which are any case invariant version of

"S",
"E",
"start",
"end",
"Finish",
"Begin",
"End",
"s",
"e",
"finish",
"begin",

The docs do not agree and some say only `S`, `E`, `start`, `end`, which others give the logner list.  

In general `how` frequency conversions should use a single list and all other forms should be deprecated.  The correct list is probably the small one {`S`, `E`, `start`, `end`}. `how` is used in other functions with different lists supported, althought he actual support is the wide list. 


### Expected Behavior

Have a small clear set of values and block others. 

### Installed Versions

<details>

INSTALLED VERSIONS

commit : 87cfe4e
python : 3.9.12.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.22621
machine : AMD64
processor : AMD64 Family 23 Model 113 Stepping 0, AuthenticAMD
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United States.1252

pandas : 1.5.0
numpy : 1.23.4
pytz : 2022.4
dateutil : 2.8.2
setuptools : 63.3.0
pip : 22.2.2
Cython : None
pytest : 7.1.3
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.8.0
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.5.0
pandas_datareader: None
bs4 : None
bottleneck : None
brotli : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.5.3
numba : None
numexpr : 2.8.3
odfpy : None
openpyxl : 3.0.10
pandas_gbq : None
pyarrow : 9.0.0
pyreadstat : 1.1.9
pyxlsb : 1.0.9
s3fs : None
scipy : 1.9.2
snappy : None
sqlalchemy : 1.4.41
tables : None
tabulate : 0.9.0
xarray : 2022.9.0
xlrd : 2.0.1
xlwt : None
zstandard : None
tzdata : None

</details>

[bashtage]

xref pandas-dev/pandas-stubs#383

[datapythonista]

+1, start and end are the ones I'd personally keep. Or maybe a bool and use end=False by default?

[olsgaard]

The validation of the how values happens in validate_end_alias

pandas/pandas/_libs/tslibs/period.pyx

Lines 2654 to 2661 in bbb1cdf

	def validate_end_alias(how: str) -> str: # Literal["E", "S"]
	how_dict = {'S': 'S', 'E': 'E',
	'START': 'S', 'FINISH': 'E',
	'BEGIN': 'S', 'END': 'E'}
	how = how_dict.get(str(how).upper())
	if how not in {'S', 'E'}:
	raise ValueError('How must be one of S or E')
	return how

Here we see that it is case insensitive, and accepts 6 different words, but if no match is found, it raises a ValueError saying only 'S' and 'E' are valid values.

So not only does the documentation for asfreq and to_timestamp need to be updated, the ValueError in validate_end_alias also needs editing (possibly along with the function itself, if it is decided to only use start and end)

I think we should keep all 6 words and update the documentation and the ValueError. While only having "start" and "end" seems cleaner, there is little to no burden in maintaining backwards compatibility, but there much burden on both users and maintainers to start a deprecation cycle.

I'd be more than happy to open a pull request to update documentation and ValueError, but I don't know how to make changes to pandas-stub, if that is also needed..