Address TODOs in eventlog encodings documentation

Summary

Location of documentation issue: (e.g. the Haddocks of base, the GHC user's guide)

The eventlog encodings sections of the GHC user's guide.

Proposed improvements or changes

Add description to the following elements:

• https://downloads.haskell.org/ghc/latest/docs/users_guide/eventlog-formats.html#event-type-GC_GLOBAL_SYNC
• https://downloads.haskell.org/ghc/latest/docs/users_guide/eventlog-formats.html#event-type-SPARK_DUD
• https://downloads.haskell.org/ghc/latest/docs/users_guide/eventlog-formats.html#event-type-SPARK_OVERFLOW
• https://downloads.haskell.org/ghc/latest/docs/users_guide/eventlog-formats.html#event-type-BIO_PROF_SAMPLE_BEGIN
• https://downloads.haskell.org/ghc/latest/docs/users_guide/eventlog-formats.html#capability-sets

Environment

Latest version of the documentation.