[analyzer][docs][NFC] Document the ento namespace in the llvm/Lexicon

Document the `ento` namespace in the Lexicon according to @nicolas17 on the
mailing list (http://lists.llvm.org/pipermail/cfe-dev/2020-August/066577.html).

The analyzer lived at different namespaces at different times.
Originally lived at the `GR` aka. (Graph Reachability)  namespace [7], later it
moved under the `ento` namespace [9].

The Static Analyzer's code lived at many other places as well:
`Analysis` -[2]-> `Checker` -[5]-> `GR` -[10]> `entoSA` -[11]-> `StaticAnalyzer`

The relevant code motion, refactor commits, cfe-dev mailing in chronological
order:
 1) 2008-03-15 Make a major restructuring of the clang tree: introduce a ...
    7a51313d
 2) 2010-01-25 Split libAnalysis into two libraries: libAnalysis and libChecker
    d6b87086
 3) 2010-12-21 Reorganization of Checker files
    http://lists.llvm.org/pipermail/cfe-dev/2010-December/012694.html
 4) 2010-12-22 Refactoring: include/clang/Checker -> include/clang/GR
    8d602a8a
 5) 2010-12-22 Refactoring: lib/Checker -> lib/GR
    2ff5ab15
 6) 2010-12-22 Refactoring: Move checkers into lib/GR/Checkers and their own
    a700e976
 7) 2010-12-22 Refactoring: Move stuff into namespace 'GR'
    ca08fba4
 8) 2010-12-22 Refactoring: Drop the 'GR' prefix.
    1696f508
 9) 2010-12-23 Rename static analyzer namespace 'GR' to 'ento'
    98857c98
10) 2010-12-23 Rename headers: 'clang/GR' 'clang/EntoSA' and update Makefile
    ef33f099
11) 2010-12-23 Chris Lattner has strong opinions about directory
    d99bd55a
12) 2010-12-24 Remove the EntoSA directories.
    9d6af532

Reviewed By: Szelethus,martong,ASDenysPetrov,xazax.hun

Differential Revision: https://reviews.llvm.org/D86446