Added some documentation to lang::java

[linuswagner]

No change to functionality done.

• added documentation to some of the lang::java functions
• deprecated getMethodSignature
• removed classPath from createM3FromMavenProject (unused parameter)
• fixed the deprecated tag in the Node file (lower case needed)

@jurgenvinju can you have a look if my descriptions are useful?
I also added references to other parts, do they work like I thought?

Also, there are some functions in AST and Core where I don't know what to write, because my understanding of Rascal/Java domain is not deep enough.
If someone wants to contribute while we're at it, feel free

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d85f086) 49% compared to head (006c815) 49%.
Report is 3 commits behind head on main.

❗ Current head 006c815 differs from pull request most recent head f47a44c. Consider uploading reports for the commit f47a44c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##              main   #1890   +/-   ##
=======================================
  Coverage       49%     49%           
- Complexity    6158    6163    +5     
=======================================
  Files          661     661           
  Lines        58695   58695           
  Branches      8546    8546           
=======================================
+ Hits         28939   28941    +2     
  Misses       27559   27559           
+ Partials      2197    2195    -2     

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[jurgenvinju]

Thanks @linuswagner; these changes look exactly like they should.