add tag formatting docs

[replay]

Defining which characters are allowed, and how to query for values containing quotes.

If you agree with the rules defined in this change, then I can also add an according validator to the TagDB

[codecov-io]

Codecov Report

Merging #2426 into master will decrease coverage by 0.05%.
The diff coverage is 52.94%.

@@            Coverage Diff             @@
##           master    #2426      +/-   ##
==========================================
- Coverage   80.11%   80.05%   -0.06%     
==========================================
  Files          85       85              
  Lines        8910     8926      +16     
  Branches     1906     1912       +6     
==========================================
+ Hits         7138     7146       +8     
- Misses       1504     1508       +4     
- Partials      268      272       +4

Impacted Files	Coverage Δ
webapp/graphite/tags/utils.py	79.41% <52.94%> (-9.05%)	⬇️

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[replay]

Added a basic validator:

(graphite-web) mst@mst-nb1:~/documents/code/go/src/github.com/graphite-project/graphite-web/webapp$ python 
Python 2.7.15rc1 (default, Nov 12 2018, 14:31:15) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from graphite.tags import utils
>>> utils.TaggedSeries.validateCharacters('abc', 'cba~')
False
>>> utils.TaggedSeries.validateCharacters('abc', 'c;ba')
False
>>> utils.TaggedSeries.validateCharacters('ab!c', 'cba')
False
>>> utils.TaggedSeries.validateCharacters('a^bc', 'cba')
False
>>> utils.TaggedSeries.validateCharacters('a=bc', 'cba')
False
>>> utils.TaggedSeries.validateCharacters('abc', 'cb=a')
True
>>> utils.TaggedSeries.validateCharacters('abc', 'cba')
True

[replay]

Just some more testing, by actually running graphite and calling the API:

mst@mst-nb1:~$ curl -XPOST 'localhost:8000/tags/tagSeries' --data-urlencode 'path=disk.used;rack=a1'
"disk.used;rack=a1"
mst@mst-nb1:~$ curl -XPOST 'localhost:8000/tags/tagSeries' --data-urlencode 'path=disk.used;rack=a1~'
{"error": "Tag/Value contains invalid characters: rack/a1~"}
mst@mst-nb1:~$ curl -XPOST 'localhost:8000/tags/tagSeries' --data-urlencode 'path=disk.used;rac!k=a1'
{"error": "Tag/Value contains invalid characters: rac!k/a1"} 
mst@mst-nb1:~$ curl -XPOST 'localhost:8000/tags/tagSeries' --data-urlencode 'path=disk.used;rack^=a1'
{"error": "Tag/Value contains invalid characters: rack^/a1"} 
mst@mst-nb1:~$ curl -XPOST 'localhost:8000/tags/tagSeries' --data-urlencode 'path=disk.used;rack====a1'
"disk.used;rack====a1"
mst@mst-nb1:~$ curl -XPOST 'localhost:8000/tags/tagSeries' --data-urlencode 'path=disk.used;rack=~a1'
{"error": "Tag/Value contains invalid characters: rack/~a1"}

[deniszh]

LGTM. Should I merge it, @DanCech ?