In an editor window of the IDE (e.g., your source code), place the cursor inside (or just after) the word I2C_WriteToSlave and press CTRL-H. You’ll get the function help for that function. If it isn’t informative enough, you can click on “View Source” to go to the library where that description/code appears.
You can also “Find in Files” with SHIFT-CTRL-F and search the libraries and samples (I use “C:\DCRABBIT_10.72*.c;.lib;.h” as my file mask – update your directory as appropriate) for all occurrences of a given function name or character sequence. I use that to find the source code of functions that don’t have function help (CTRL-H) available.